Quiz: Measurement Uncertainties and Periodicity — 63 questions

1. Which section of a Grade 11 experimental write-up uses observations to address the purpose of the investigation?

A. The theory

B. The apparatus list

C. The uncertainty analysis

D. The conclusion

Answer: (D)

Explanation

The conclusion addresses the investigation’s purpose by using the observed results. The theory instead predicts or interprets those results, so it serves a different role.

2. When may students remove required safety goggles during a laboratory activity?

A. When the teacher leaves the laboratory area

B. When they begin writing the results section

C. After all groups have completed the activity

D. After their own group has finished collecting data

Answer: (C)

Explanation

Required safety equipment must remain in use throughout the activity and may be removed after every group has completed it. Finishing one group’s work does not remove hazards for others.

3. What is the safe procedure for diluting an acid with water?

A. Add the acid and water simultaneously

B. Pour the acid slowly into the water

C. Mix equal volumes in a sealed container

D. Pour the water slowly into the acid

Answer: (B)

Explanation

Acid must be poured into water because this procedure reduces the risk of dangerous splashing from heat released during dilution. Pouring water into acid can cause the mixture to heat and erupt.

4. Which description identifies an element?

A. A non-uniform combination containing multiple physical phases

B. A pure substance that cannot be chemically broken down into simpler substances

C. A pure substance made from substances that can be chemically decomposed

D. A combination whose components retain their individual properties

Answer: (B)

Explanation

An element is a pure substance that ordinary chemical methods cannot decompose into simpler substances. A compound can be chemically decomposed, while a mixture contains substances that retain their own properties.

5. A sample contains two pure substances, and each retains its own physical and chemical properties. How should the sample be classified?

A. As a compound

B. As a chemical change

C. As an element

D. As a mixture

Answer: (D)

Explanation

A mixture combines pure substances without eliminating their individual physical and chemical properties. A compound forms a new pure substance whose components are chemically combined.

6. A liquid sample is uniform throughout and contains one phase. What classification best describes it?

A. An element with a constant atomic number

B. A heterogeneous mixture with several phases

C. A homogeneous mixture, or solution

D. A compound formed by chemical bonding

Answer: (C)

Explanation

A homogeneous mixture is uniform throughout, contains one phase, and is called a solution. A heterogeneous mixture is non-uniform and contains more than one phase.

7. Which observation indicates that a chemical change has occurred rather than a physical change?

A. The sample is divided into smaller pieces with the same identity

B. The substance changes shape while retaining its composition

C. A new substance forms with different composition and properties

D. The material changes phase without changing chemical identity

Answer: (C)

Explanation

A chemical change produces at least one new substance with a different composition and properties. Changes in shape, phase, or size can occur without changing chemical identity.

8. What does the atomic number of an atom represent?

A. The number of protons in its nucleus

B. The weighted average mass of its naturally occurring isotopes

C. The number of neutrons outside its nucleus

D. The total number of protons and neutrons in its nucleus

Answer: (A)

Explanation

Atomic number is defined by the number of protons in the nucleus. The total of protons and neutrons is the mass number, not the atomic number.

9. Two atoms belong to the same element but have different numbers of neutrons. What is their relationship?

A. They are isotopes of the same element

B. They are compounds with different compositions

C. They are radioisotopes of different elements

D. They are ions with different proton numbers

Answer: (A)

Explanation

Isotopes are atoms of the same element with different neutron numbers. They are radioisotopes only when they are radioactive, and differing neutron counts do not make them different elements.

10. An element has two naturally occurring isotopes with masses $m_1$ and $m_2$ and percentage abundances $p_1$ and $p_2$. Which expression calculates its average atomic mass?

A. $\frac{p_1m_1+p_2m_2}{100}$

B. $\frac{m_1+m_2}{p_1+p_2}$

C. $\frac{100(m_1+m_2)}{p_1p_2}$

D. $\frac{p_1+p_2}{m_1+m_2}$

Answer: (A)

Explanation

Average atomic mass is found by multiplying each isotopic mass by its percentage abundance, adding the products, and dividing by 100. Dividing masses by abundances does not produce the required weighted average.

11. Which statement correctly defines the significant digits in a measured quantity?

A. They include all digits displayed after the decimal point.

B. They include every digit in an exact counted number.

C. They include only the digits known with complete certainty.

D. They include all certain digits and one estimated digit.

Answer: (D)

Explanation

Significant digits contain the digits known with certainty plus one uncertain or estimated digit. Exact counted numbers differ because they have an unlimited number of significant digits.

12. How many significant digits are in the number 0.004050?

A. Two significant digits

B. Four significant digits

C. Three significant digits

D. Five significant digits

Answer: (B)

Explanation

The leading zeros locate the decimal point and are not significant, while the captive zero and specified trailing zero count, giving four significant digits. Treating every displayed zero as insignificant would incorrectly omit the captive and trailing zeros.

13. What result should be reported for $12.37+0.6$ using the addition rule for significant digits?

A. $12.97$

B. $12.9$

C. $12.970$

D. $13.0$

Answer: (D)

Explanation

Addition is rounded to the fewest decimal places, and 0.6 has one decimal place, so the result is reported as 13.0. Reporting 12.97 retains more decimal places than the least precise quantity supports.

14. What result should be reported for $3.24\times1.5$ using the multiplication rule for significant digits?

A. $4.860$

B. $4.9$

C. $5$

D. $4.86$

Answer: (B)

Explanation

The product is 4.86, but the factor 1.5 has two significant digits, so the result must be reported as 4.9. The unrounded value has three significant digits and therefore conveys unsupported precision.

15. A length is measured as $8.42\ \mathrm{cm}\pm0.03\ \mathrm{cm}$. What does the absolute uncertainty represent?

A. An uncertainty of 8.45 cm in the measured length

B. An uncertainty of 8.42 cm in the measuring instrument

C. An uncertainty of 0.03 percent in the measured length

D. An uncertainty of 0.03 cm in the measured length

Answer: (D)

Explanation

Absolute uncertainty is expressed in the same unit as the measured quantity, so it is 0.03 cm here. Percentage uncertainty is dimensionless and would require dividing the absolute uncertainty by the measured value.

16. A ruler has scale divisions of 2 mm. What uncertainty is generally assigned to a direct measurement made with it?

A. $\pm2\ \mathrm{mm}$

B. $\pm0.5\ \mathrm{mm}$

C. $\pm1\ \mathrm{mm}$

D. $\pm4\ \mathrm{mm}$

Answer: (C)

Explanation

For a direct scale measurement, the uncertainty is generally half the smallest division, so half of 2 mm is 1 mm. The full division is a resolution, not the usual assigned uncertainty under this rule.

17. What is the percentage uncertainty for a measurement of $50.0\ \mathrm{g}\pm1.0\ \mathrm{g}$?

A. $50.0\%$

B. $0.02\%$

C. $2.0\%$

D. $51.0\%$

Answer: (C)

Explanation

Using $\frac{\text{absolute uncertainty}}{\text{measured value}}\times100\%$ gives $\frac{1.0}{50.0}\times100\%=2.0\%$. The absolute uncertainty itself is not a percentage until it is divided by the measured value.

18. Two lengths are measured as $4.2\pm0.1\ \mathrm{cm}$ and $2.1\pm0.2\ \mathrm{cm}$. What absolute uncertainty should be assigned to their sum?

A. $\pm0.02\ \mathrm{cm}$

B. $\pm0.2\ \mathrm{cm}$

C. $\pm0.3\ \mathrm{cm}$

D. $\pm0.1\ \mathrm{cm}$

Answer: (C)

Explanation

For addition, the absolute uncertainties are added, giving 0.1 cm + 0.2 cm = 0.3 cm. Adding percentage uncertainties would apply to multiplication or division rather than this sum.

19. What instrument uncertainty is appropriate for a ruler calibrated in millimetres?

A. $\pm0.05\ \mathrm{mm}$

B. $\pm1.0\ \mathrm{mm}$

C. $\pm5.0\ \mathrm{mm}$

D. $\pm0.5\ \mathrm{mm}$

Answer: (D)

Explanation

A millimetre-calibrated ruler has an instrument uncertainty of half a millimetre, or ±0.5 mm. This uncertainty describes the instrument's precision and is not the measured length itself.

20. Four measurements of the same quantity are 5.0, 4.8, 5.3, and 4.9. What uncertainty should be assigned to their average using half the range?

A. $\pm5.00$

B. $\pm0.25$

C. $\pm0.10$

D. $\pm0.50$

Answer: (B)

Explanation

The overall range is 5.3 − 4.8 = 0.5, and half of that range is 0.25, so the uncertainty is ±0.25. The full range describes the spread, whereas the stated uncertainty is half that spread.

21. A mass is reported as $80\ \mathrm{g}\pm2\ \mathrm{g}$. What is its percentage uncertainty?

A. $2.5\%$

B. $82\%$

C. $40\%$

D. $0.025\%$

Answer: (A)

Explanation

Percentage uncertainty is calculated as $\frac{u}{x}\times100=\frac{2}{80}\times100=2.5\%$. The absolute uncertainty remains 2 g, while the percentage form is dimensionless.

22. Which description correctly identifies an absolute uncertainty?

A. A ratio of uncertainty to measurement expressed as a percentage

B. An uncertainty expressed in the measured quantity's unit

C. The difference between the maximum and minimum readings

D. The central value obtained by averaging repeated readings

Answer: (B)

Explanation

Absolute uncertainty is measured directly and reported in the same unit as the quantity being measured. A ratio expressed as a percentage is percentage uncertainty, while the range and average describe spread and central value.

23. When adding or subtracting two measured quantities, how should their uncertainties be combined?

A. By adding their absolute uncertainties

B. By averaging their percentage uncertainties

C. By multiplying their absolute uncertainties

D. By adding their percentage uncertainties

Answer: (A)

Explanation

Addition and subtraction require the absolute uncertainties to be added because the possible measurement errors combine directly in the same units. Percentage uncertainties are used for products and quotients, not for sums and differences.

24. A measurement of $4.0 \pm 0.2$ is multiplied by a measurement of $2.0 \pm 0.2$. What uncertainty rule applies to this product?

A. Average the absolute uncertainties of the factors

B. Subtract the percentage uncertainty of the smaller factor

C. Add the percentage uncertainties of the factors

D. Add the absolute uncertainties of the factors

Answer: (C)

Explanation

For multiplication, the percentage uncertainties of the factors are added to find the percentage uncertainty of the product. Adding absolute uncertainties is the rule for addition and subtraction, so it does not apply here.

25. What did Mendeleev propose in 1869 about the arrangement of elements?

A. Elements ordered by increasing atomic mass show recurring similar properties

B. Elements placed in horizontal rows have matching valence shells

C. Elements ordered by atomic number form groups of eight

D. Elements arranged in triads show identical chemical behavior

Answer: (A)

Explanation

Mendeleev stated in 1869 that arranging elements by increasing atomic mass revealed similar properties at regular intervals. Groups of three describe Dobereiner’s triads, while groups of eight describe Newland’s observation.

26. Which description correctly distinguishes a group from a period in the periodic table?

A. A group follows atomic mass, while a period follows atomic number

B. A group is vertical, while a period is horizontal

C. A group is horizontal, while a period is vertical

D. A group contains three elements, while a period contains eight

Answer: (B)

Explanation

A group, or family, is a vertical column whose elements have similar properties, whereas a period is a horizontal row. Triads and groups of eight refer to historical classification ideas rather than the definitions of groups and periods.

27. Which historical classification consisted of groups of three similar elements identified in 1829?

A. Mendeleev’s periodic arrangement

B. Johann Dobereiner’s triads

C. The modern atomic-number sequence

D. John Newland’s octaves

Answer: (B)

Explanation

Johann Dobereiner identified triads, or groups of three similar elements, in 1829. Newland’s work involved recurring similarities after every eighth element, not groups of three.

28. Which electrons are classified as valence electrons?

A. Electrons in filled inner levels that cause shielding

B. Electrons located in the nucleus of an atom

C. Electrons released when an atom becomes an ion

D. Electrons in the highest occupied energy level

Answer: (D)

Explanation

Valence electrons occupy the highest energy level that contains electrons and strongly influence chemical behavior. Inner-level electrons contribute to shielding, but they are not valence electrons.

29. What happens when an alkali metal from Family IA reacts with water?

A. It becomes a noble gas without producing gas

B. It reacts to form hydrogen gas

C. It forms a halogen compound and absorbs hydrogen

D. It loses its valence electron but produces oxygen gas

Answer: (B)

Explanation

Family IA alkali metals have one valence electron and react with water to produce hydrogen gas. Their characteristic reaction does not produce oxygen, and losing an electron does not turn them into noble gases.

30. Which statement correctly describes the halogens in Family VIIA?

A. They are reactive metals, with fluorine less reactive than iodine

B. They are alkali metals, with all members having one valence electron

C. They are stable noble gases, with iodine more reactive than fluorine

D. They are reactive nonmetals, with fluorine more reactive than iodine

Answer: (D)

Explanation

The halogens are reactive nonmetals, and among the listed members fluorine is the most reactive while iodine is the least reactive. Noble gases belong to Family 0 or VIII, whereas alkali metals belong to Family IA.

31. How does a sodium atom become a sodium ion with a +1 charge?

A. It loses two electrons and obtains helium’s electron structure

B. It loses one electron and obtains neon’s electron structure

C. It shares one electron and obtains chlorine’s electron structure

D. It gains one electron and obtains neon’s electron structure

Answer: (B)

Explanation

A sodium atom forms a +1 ion by losing one electron, leaving it with the same electron structure as neon. Gaining an electron would give sodium a negative charge rather than a positive one.

32. What distinguishes a periodic trend from an isolated property of an element?

A. A periodic trend is a gradual, consistent change across periods or groups.

B. A periodic trend is a sudden change found in one individual element.

C. A periodic trend is a property that varies randomly throughout the table.

D. A periodic trend is a measurement that applies only to atomic mass.

Answer: (A)

Explanation

A periodic trend describes a gradual and consistent change in properties within periods or groups. An isolated property may describe one element without showing a systematic pattern across the periodic table.

33. What does atomic radius measure?

A. The number of occupied energy levels surrounding an atomic nucleus

B. The distance from the nucleus to the most probable location of the outermost electron

C. The energy required to remove the outermost electron from an isolated atom

D. The tendency of an atom to attract an electron in a chemical bond

Answer: (B)

Explanation

Atomic radius refers to the distance from the nucleus to the most probable location of the outermost electron. The energy needed to remove an electron is ionization energy, not atomic radius.

34. Why does atomic radius generally increase down a family of elements?

A. The nucleus becomes smaller as additional electron shells are added to the atom.

B. The outer electron experiences less shielding because fewer inner electrons are present.

C. The nuclear charge decreases while the number of occupied energy levels remains constant.

D. Additional energy levels and increased shielding place the outer electron farther from the nucleus.

Answer: (D)

Explanation

Atoms gain energy levels down a family, and increased shielding places the outer electron farther from the nucleus. A larger nuclear charge across a period instead tends to decrease atomic radius when the number of energy levels stays the same.

35. How does ionization energy generally change across the periodic table?

A. It decreases down a family and increases from left to right across a period.

B. It increases both down a family and from left to right across a period.

C. It decreases both down a family and from left to right across a period.

D. It increases down a family and decreases from left to right across a period.

Answer: (A)

Explanation

Ionization energy generally decreases down a family and increases from left to right across a period. Greater distance and shielding lower the removal energy down a family, while increasing nuclear charge raises it across a period.

36. Which description correctly contrasts ionic and covalent bonding?

A. Ionic bonding transfers electrons between a metal and a nonmetal, whereas covalent bonding shares pairs between nonmetals.

B. Ionic bonding shares electron pairs between nonmetals, whereas covalent bonding transfers electrons from metals.

C. Ionic bonding transfers protons between atoms, whereas covalent bonding shares neutrons between nonmetals.

D. Ionic bonding shares electrons between two metals, whereas covalent bonding transfers pairs to a nonmetal.

Answer: (A)

Explanation

Ionic bonding involves electron transfer from a metal to a nonmetal, while covalent bonding involves shared electron pairs between nonmetals. Covalent bonds are therefore not defined by electron transfer.

37. What occurs when two atoms form a single covalent bond?

A. They share three pairs of electrons.

B. They share two pairs of electrons.

C. They share one pair of electrons.

D. They transfer one electron from one atom to another.

Answer: (C)

Explanation

A single covalent bond consists of one shared pair of electrons between two atoms. Sharing two or three pairs produces double or triple bonds, respectively.

38. A bond between two atoms has an electronegativity difference of 2.0. How is this bond classified by convention?

A. As a polar covalent bond because the difference remains below 2.5

B. As a metallic bond because the atoms have unequal electronegativities

C. As an ionic bond because the difference exceeds 1.7

D. As a nonpolar covalent bond because both atoms share electrons

Answer: (C)

Explanation

By convention, an electronegativity difference greater than 1.7 indicates an ionic bond. A smaller difference would generally correspond to a covalent bond with polarity depending on its magnitude.

39. According to VSEPR theory, why do electron pairs arrange themselves as far apart as possible?

A. Electron pairs repel one another in the valence shell.

B. Electron pairs move closer together to maximize orbital overlap.

C. Electron pairs attract one another toward the central atom.

D. Electron pairs occupy positions determined by atomic mass.

Answer: (A)

Explanation

VSEPR theory explains molecular arrangements by treating valence-shell electron pairs as mutually repelling regions. Separating them as far as possible minimizes these repulsions.

40. Which sequence correctly matches the number of electron pairs with their most probable arrangements?

A. Two pairs are tetrahedral, three are linear, and four are trigonal planar.

B. Two pairs are linear, three are trigonal planar, and four are tetrahedral.

C. Two pairs are trigonal pyramidal, three are linear, and four are tetrahedral.

D. Two pairs are trigonal planar, three are tetrahedral, and four are linear.

Answer: (B)

Explanation

VSEPR predicts linear, trigonal planar, and tetrahedral arrangements for two, three, and four electron pairs, respectively. Trigonal planar corresponds to three pairs, whereas tetrahedral corresponds to four.

41. Which set of ideal bond angles correctly corresponds to linear, trigonal planar, and tetrahedral geometries?

A. 180°, 109.5°, and 120°, respectively

B. 109.5°, 180°, and 120°, respectively

C. 120°, 109.5°, and 180°, respectively

D. 180°, 120°, and 109.5°, respectively

Answer: (D)

Explanation

Linear geometry has an ideal angle of 180°, trigonal planar geometry has 120°, and tetrahedral geometry has 109.5°. Confusing the first two geometries reverses the characteristic angles.

42. What molecular geometry and bond angle does VSEPR predict for methane, CH₄?

A. A trigonal pyramidal geometry with bond angles of 109.5°

B. A tetrahedral geometry with bond angles of 109.5°

C. A linear geometry with bond angles of 180°

D. A trigonal planar geometry with bond angles of 120°

Answer: (B)

Explanation

Methane has four shared electron pairs around carbon, so VSEPR predicts a tetrahedral arrangement with bond angles of 109.5°. Trigonal planar and linear geometries correspond to three and two electron pairs, respectively.

43. Which sequence correctly ranks valence-shell electron-pair repulsions from strongest to weakest?

A. Lone pair–lone pair, lone pair–shared pair, shared pair–shared pair

B. Shared pair–shared pair, lone pair–shared pair, lone pair–lone pair

C. Lone pair–shared pair, shared pair–shared pair, lone pair–lone pair

D. Shared pair–shared pair, lone pair–lone pair, lone pair–shared pair

Answer: (A)

Explanation

Lone pairs occupy more space around the central atom, so their mutual repulsion is strongest, followed by lone pair–shared pair and shared pair–shared pair interactions. Treating shared-pair repulsion as strongest reverses the VSEPR ranking.

44. Why does ammonia, NH₃, have a trigonal pyramidal molecular shape with a bond angle of about $107.3^\circ$?

A. Two lone pairs on nitrogen spread three N–H shared pairs into a bent arrangement

B. One lone pair on nitrogen pushes three N–H shared pairs closer together

C. One lone pair on each hydrogen pushes the nitrogen atom above a planar arrangement

D. No lone pairs on nitrogen allow three N–H bonds to form a flat triangle

Answer: (B)

Explanation

Ammonia has one lone pair on nitrogen and three shared pairs, producing a trigonal pyramidal molecular shape and compressing the bond angle to about $107.3^\circ$. Methane is tetrahedral because its carbon has no lone pair, not because it has two lone pairs.

45. Which molecular shape and bond angle best describe water, H₂O?

A. Angular, with a bond angle of about $104.5^\circ$

B. Linear, with a bond angle of about $180^\circ$

C. Trigonal pyramidal, with a bond angle of about $107.3^\circ$

D. Tetrahedral, with a bond angle of about $109.5^\circ$

Answer: (A)

Explanation

Water is angular because two lone pairs on oxygen repel the two shared pairs, giving a bond angle of about $104.5^\circ$. Ammonia is trigonal pyramidal because it has one lone pair rather than two.

46. How are the shared pairs in a double bond represented in VSEPR theory?

A. They occupy one two-electron orbital and count as one electron domain

B. They occupy two four-electron orbitals and count as four electron domains

C. They occupy two separate two-electron orbitals and count as two electron domains

D. They occupy one four-electron orbital and count as one electron domain

Answer: (D)

Explanation

For VSEPR purposes, the two shared pairs in a double bond occupy one four-electron orbital, so the bond counts as one electron domain. Treating the electrons as two separate orbitals incorrectly doubles the domain count.

47. Which order correctly ranks orbital repulsions from a multiple bond as strongest to weakest?

A. Triple-bond orbital, double-bond orbital, single-bond orbital

B. Double-bond orbital, single-bond orbital, triple-bond orbital

C. Triple-bond orbital, single-bond orbital, double-bond orbital

D. Single-bond orbital, triple-bond orbital, double-bond orbital

Answer: (A)

Explanation

A triple-bond orbital produces the greatest repulsion, followed by a double-bond orbital and then a single-bond orbital. The ranking reflects the increasing number of shared electron pairs concentrated in the bonding orbital.

48. What molecular geometry and bond angles are expected for formaldehyde because its double-bond orbital repels the single-bond orbitals?

A. Angular, with an H–C–H angle of $104.5^\circ$ and H–C–O angles of $107.3^\circ$

B. Basically trigonal planar, with an H–C–H angle of $118^\circ$ and H–C–O angles of $121^\circ$

C. Tetrahedral, with an H–C–H angle of $109.5^\circ$ and H–C–O angles of $109.5^\circ$

D. Linear, with an H–C–H angle of $180^\circ$ and equal H–C–O angles of $90^\circ$

Answer: (B)

Explanation

Formaldehyde is basically trigonal planar, and the stronger double-bond orbital widens the H–C–O angles to $121^\circ$ while compressing the H–C–H angle to $118^\circ$. Carbon dioxide is linear because its central carbon is surrounded by two orbitals, not three.

49. Why is carbon dioxide, O=C=O, linear around the carbon atom?

A. The carbon atom has one lone pair occupying a third orbital between two bonds

B. The oxygen atoms repel each other into a bent arrangement around carbon

C. Each double bond counts as two separate electron domains around carbon

D. The carbon atom is surrounded by four electron pairs occupying two orbitals

Answer: (D)

Explanation

In carbon dioxide, the four shared pairs are arranged as two electron domains because each double bond occupies one orbital, producing a linear geometry. Counting the two double bonds as four separate domains would incorrectly predict a bent structure.

50. What does the valence of an element express in a compound?

A. The number of neutrons contained in its most common isotope

B. The number of electrons occupying its outermost shell

C. The mass contribution it makes to a compound formula

D. Its combining capacity, represented by a sign and a numerical value

Answer: (D)

Explanation

Valence expresses an element’s combining capacity in a compound and is written with a sign and numerical value. The number of valence electrons instead refers to electrons in the outer shell, which is a different concept.

51. Which set contains the elements that normally occur as diatomic molecules?

A. He₂, Ne₂, Ar₂, Kr₂, Xe₂, Rn₂, and Og₂

B. N₂, H₂, O₂, F₂, Cl₂, Br₂, and I₂

C. Li₂, Be₂, Na₂, Mg₂, K₂, Ca₂, and Ba₂

D. C₂, Si₂, Ge₂, Sn₂, Pb₂, B₂, and Al₂

Answer: (B)

Explanation

The seven elements that normally occur as diatomic molecules are nitrogen, hydrogen, oxygen, fluorine, chlorine, bromine, and iodine. Noble gases and most metals are not represented by this standard set of diatomic elemental molecules.

52. Which valence assignment correctly matches the periodic-table groups?

A. Group 1: zero; Group 2: +3; Group 13: +1; Group 18: +2

B. Group 1: +1; Group 2: +2; Group 13: +3; Group 18: zero

C. Group 1: +2; Group 2: +1; Group 13: zero; Group 18: +3

D. Group 1: +3; Group 2: zero; Group 13: +2; Group 18: +1

Answer: (B)

Explanation

The usual valences are +1 for Group 1, +2 for Group 2, +3 for Group 13, and zero for Group 18 noble gases. Confusing group number with valence produces the alternative assignments.

53. Which description identifies a binary compound?

A. A compound made of two different elements

B. A compound formed from two identical atoms

C. A compound containing hydrogen and oxygen only

D. A compound containing two molecules of one element

Answer: (A)

Explanation

A binary compound contains exactly two elements, regardless of the total number of atoms in its formula. HCN is not binary because it contains hydrogen, carbon, and nitrogen.

54. What is the correct procedure for writing a binary ionic formula from the compound name?

A. Add the valences together and use the sum as the compound's subscript

B. Copy each element's valence directly as its subscript in named order

C. Place the metal second and make the two valences equal

D. Place the elements in named order, balance charges to zero, and omit a subscript of one

Answer: (D)

Explanation

A binary ionic formula must have total positive and negative charges that sum to zero, with the elements written in the named order. Simply copying valences can produce a formula with a nonzero net charge.

55. Why is a Roman numeral included in the name of some binary ionic compounds?

A. It identifies the number of negative ions present

B. It gives the number of atoms in the entire compound

C. It indicates that the compound contains molecular prefixes

D. It identifies the valence of a multivalent element

Answer: (D)

Explanation

A multivalent element can form ions with different valences, so the Roman numeral specifies which valence is present. The numeral does not count atoms or identify molecular prefixes.

56. What happens when a base dissolves in water?

A. It releases oxygen molecules, O₂

B. It produces hydroxyl ions, OH⁻

C. It converts all dissolved ions into hydrogen gas

D. It produces hydronium ions, H₃O⁺

Answer: (B)

Explanation

A base contains a hydroxyl radical or produces hydroxyl ions, OH⁻, in water. Producing hydronium ions is characteristic of an acid rather than a base.

57. Which name pattern identifies a binary acid?

A. Per- followed by the element root and -ic acid

B. The metal name followed by hydrogen acid

C. The element root followed by -ous acid

D. Hydro- followed by the element root and -ic acid

Answer: (D)

Explanation

A binary acid contains hydrogen and one other element and is named with the hydro-…-ic acid pattern. Oxy-acids contain oxygen and follow different naming relationships.

58. Which acid is correctly matched with its formula?

A. Phosphoric acid — H₃PO₄

B. Sulphuric acid — HNO₃

C. Nitric acid — H₃BO₃

D. Carbonic acid — H₂SO₄

Answer: (A)

Explanation

Phosphoric acid has the formula H₃PO₄ and is one of the principal oxy-acids. H₂SO₄ is sulphuric acid, H₃BO₃ is boric acid, and HNO₃ is nitric acid.

59. Compared with the parent -ic oxy-acid, how many fewer oxygen atoms does a hypo-…-ous acid contain?

A. One fewer oxygen atom

B. Two more oxygen atoms

C. Two fewer oxygen atoms

D. The same number of oxygen atoms

Answer: (C)

Explanation

The hypo-…-ous form has two fewer oxygen atoms than the parent -ic acid. The -ous form has one fewer oxygen, while the per-…-ic form has one more.

60. A chemical reaction occurs in a closed system, and the reactants have a total mass of 125 grams. What total mass should the products have according to conservation of mass?

A. 125 grams

B. More than 125 grams because energy is released

C. A mass determined by the compound's fixed element ratio

D. Less than 125 grams because gases may form

Answer: (A)

Explanation

Conservation of mass states that matter is neither created nor destroyed, so the products must have the same total mass as the reactants. A fixed element ratio describes definite composition, not the total mass comparison in a reaction.

61. Two samples of a pure compound contain different total amounts but have the same mass ratio of their elements. Which law does this illustrate?

A. The law of double replacement

B. The law of conservation of mass

C. The law of definite composition

D. The law of multiple proportions

Answer: (C)

Explanation

The law of definite composition states that every pure compound has a constant composition by mass, so its elements occur in fixed mass proportions. Conservation of mass instead compares total reactant and product masses during a change.

62. Two elements form two different compounds. In both compounds, the same mass of the first element combines with masses of the second element in a ratio of 1:2. Which law is demonstrated?

A. The law of multiple proportions

B. The law of conservation of mass

C. The law of synthesis

D. The law of definite composition

Answer: (A)

Explanation

The law of multiple proportions applies when two elements form more than one compound and the different combining masses show a simple whole-number ratio. Definite composition concerns the fixed ratio within one pure compound.

63. Which reaction type is represented by the pattern $XY\longrightarrow X+Y$?

A. Single replacement

B. Double replacement

C. Decomposition

D. Synthesis

Answer: (C)

Explanation

A decomposition reaction breaks one compound into two or more substances, matching the pattern shown. Synthesis has the reverse pattern, combining multiple substances into one product.