Module 5: Chaper 20 - Acids, Bases and pH.

Question 1

What is a Brønsted-Lowry Acid?

Answer 1

A proton donor.

Question 2

What is a Brønsted-Lowry Base?

Answer 2

A proton acceptor.

Question 3

How are conjugate acid-base pairs related?

Answer 3

The conjugate acid donates a proton (H⁺) to make the conjugate base; the base accepts the proton (H⁺) to make the acid.

Question 4

What is the Conjugate Base of these below?
* HNO₃.
* H₂0.
* H₂SO₄.
* HCl.

Answer 4

• NO₃^-.
• OH^-.
• HSO₄^-.
• Cl^-.

Question 5

What is the Conjugate Acid of these below?
* OH^-.
* H₂0.
* SO₄^2-.
* Br^-.

Answer 5

• H₂0.
• H₃O⁺.
• HSO₄^-.
• HBr.

Question 6

Explain what each of the below are and give 1 example of each.
1. Monobasic Acid.
2. Dibasic Acid.
3. Tribasic Acid.

Answer 6

1. An acid which only has 1 H⁺ ion to donate to its conjugate base (HCl).
2. An acid which only has 2 H⁺ ions to donate to its conjugate base (H₂SO₄).
3. An acid which only has 3 H⁺ ions to donate to its conjugate base (H₃PO₄).

Question 7

What is the DEFINITION of a Weak Acid? Give a general equation for this?

Answer 7

• A weak acid only partially dissociates it’s H⁺ ions in aqueous solution.
• [HA] ⇌ [H⁺] + [A^-] which leads to [H⁺] ∼ [A^-].

Question 8

What is the DEFINITION of a Strong Acid?

Answer 8

• A strong acid fully dissociates all of it’s H⁺ ions in aqueous solution.
• [HA] ∼ [H⁺].

Question 9

Write the Ka expression for a general weak acid HA.

Answer 9

Ka = [H⁺][A^-]/ [HA].

Question 10

Which equation gives pH from [H⁺]?

Answer 10

pH = -log[H⁺].

Question 11

Which equation gives pKa from Ka?

Answer 11

pKa = -log(Ka).

Question 12

Which equation gives [H⁺] from pH?

Answer 12

[H⁺] = 10^-pH.

Question 13

What simplified Ka expression can we use for weak acids?

Answer 13

Ka = [H⁺]²/ [HA].

Question 14

How does the simplified Ka expression Ka = [H⁺]²/ [HA] rearrange to solve for [H⁺]?

Answer 14

[H⁺] = √(Ka x [HA]).

Question 15

What is the expression for the ionic product of water?

Answer 15

Kw = [H⁺][OH^-].

Question 16

What is the value of Kw for neutral pH7 at 25°C?

Answer 16

1.00 x 10^-14 mol² dm^-6.

Question 17

Which equation gives Ka from pKa?

Answer 17

Ka = 10^-pKa.

Question 18

Does a larger Ka value mean a stronger or weaker acid?

Answer 18

Stronger - The acid dissociation equilibrium lies further to the right.

Question 19

Does a larger pKa value mean a stronger or weaker acid?

Answer 19

Weaker - Trend follows the reverse of Ka due to -ve sign in -log(Ka).

Question 20

What two approximations are made when calculating pH of weak acids?

Answer 20

• [HA]eqm = [HA]start.
• [H⁺] = [A^-].

Question 21

When does the approximation that [H⁺] = [A^-] for a weak acid break down?

Answer 21

For very dilute or very weak solutions of acid as the H⁺ from water dissociation is significant compared to the H⁺ from the acid.

Question 22

What is the Kw expression in pure water?

Answer 22

[H⁺] = [OH^-] so Kw = [H⁺]², therefore [H⁺] = √Kw.