Flashcards: Water, pH and Buffer Solutions — 37 cards

How many hydrogen and oxygen atoms are in a water molecule?

Two hydrogen atoms and one oxygen atom.

What type of bonds link atoms in a water molecule?

Covalent bonds.

What is the H–O–H bond angle in a water molecule?

104.5°.

What is the overall charge of a water molecule?

Globally neutral charge.

Which atom in water is more electronegative, oxygen or hydrogen?

Oxygen is more electronegative than hydrogen.

What are the partial charges on hydrogen and oxygen in water?

Hydrogen has +0.41 and oxygen has −0.82 partial charges.

What is a hydrogen bond?

An interaction between partial positive and partial negative charges of molecules.

How many hydrogen bonds can one water molecule theoretically form?

Four hydrogen bonds.

What is the approximate energy of a hydrogen bond in water?

About 19 kJ·mol−1.

How does hydrogen bonding differ between ice and liquid water?

Ice has four stable hydrogen bonds per molecule; liquid water has fewer, short-lived bonds.

How do hydrogen bonds affect salt crystals in water?

They organize oppositely charged ions and promote crystal dissolution.

Which groups can form hydrogen bonds with water?

Hydroxyl, carbonyl, carboxyl, and amine groups can form hydrogen bonds.

Which group cannot form hydrogen bonds with water?

The methyl group cannot form hydrogen bonds with water.

What characterizes hydrophilic molecules regarding water?

They form hydrogen bonds with water and tend to be soluble.

What characterizes hydrophobic molecules regarding water?

They are nonpolar, do not form hydrogen bonds, and separate from water.

Where do hydrophobic interactions commonly occur?

In oils, phospholipid membranes, and nonpolar solvents.

What structures can hydrophobic interactions lead to?

They can lead to micelles, liposomes, or bilayers.

What is the water ionization reaction equation?

$\mathrm{H_2O \rightleftharpoons H^+ + OH^-}$

What is the ionic product constant of water at 25°C?

$K_e = [\mathrm{H^+}][\mathrm{OH^-}] = 10^{-14}\,(\mathrm{mol/L})^2$

What are the concentrations of H+ and OH− in pure water?

Both are $10^{-7}\,\mathrm{mol/L}$

How is pH defined in terms of hydrogen ion concentration?

$\mathrm{pH} = -\log_{10}[\mathrm{H^+}]$

What is the pH of pure water?

7

When is a solution considered neutral?

When $[\mathrm{H^+}] = [\mathrm{OH^-}]$

When is a solution acidic?

When $[\mathrm{H^+}] > [\mathrm{OH^-}]$

When is a solution basic?

When $[\mathrm{H^+}] < [\mathrm{OH^-}]$

What distinguishes a strong acid or base from a weak acid–base pair?

A strong acid or base dissociates completely, a weak pair dissociates incompletely.

What happens when 10⁻² mol·L⁻¹ hydrochloric acid is added to a solution?

It produces 10⁻² mol·L⁻¹ H⁺ and results in pH 2.

Why does hydrochloric acid produce pH 2 at 10⁻² mol·L⁻¹ concentration?

Because hydrochloric acid dissociates completely.

What is the formula for the acid dissociation constant $K_a$ of a weak acid–base pair?

$K_a=\frac{[\mathrm{H^+}][\mathrm{base}]}{[\mathrm{acid}]}$

What is the Henderson-Hasselbalch equation for pH of a weak acid–base pair?

$\mathrm{pH}=\mathrm{p}K_a+\log\frac{[\mathrm{base}]}{[\mathrm{acid}]}$

How is $\mathrm{p}K_a$ defined in terms of $K_a$?

$\mathrm{p}K_a=-\log K_a$

What does the pKa represent in a weak acid and its conjugate base system?

The pH where their concentrations are equal.

What is a buffer solution?

A solution whose pH changes little when acid or base is added.

What is the initial pH of an acetate/acetic acid buffer with equal 50×10⁻³ mol·L⁻¹ components and pKa 4.85?

The initial pH is 4.85.

How does adding 10⁻² mol·L⁻¹ HCl affect acetate and acetic acid concentrations in the buffer?

Acetate decreases to 40×10⁻³ mol·L⁻¹ and acetic acid increases to 60×10⁻³ mol·L⁻¹.

What is the pH after adding 10⁻² mol·L⁻¹ HCl to the acetate/acetic acid buffer?

The pH becomes 4.67.

When is a buffer most effective?

When its pH is close to the pKa of its weak acid–base pair.