3. Acids and Bases

Amino acids exist predominantly in one of the following forms. Which is it? Explain your answer.

Which of the four reactions has the most favorable equilibrium constant?

1.

b. Determine the exact pKa values, using a calculator.

c. Which is the strongest acid?

1. nitrous acid (HNO₂), K_a = 4.0 × 10⁻⁴

2. nitric acid (HNO₃), K_a = 22

3. bicarbonate (HCO₃⁻), K_a = 6.3 × 10⁻¹¹

4. hydrogen cyanide (HCN), K_a = 7.9 × 10⁻¹⁰

5. formic acid (HCOOH), K_a = 2.0 × 10⁻⁴

6. phosphoric acid (H₃PO₄), K_a = 2.1

Given the K_a values, estimate the pK_a value of each of the following acids without using a calculator (that is, is it between 3 and 4, between 9 and 10, and so on?):

1. nitrous acid (HNO₂), K_a = 4.0 × 10⁻⁴

2. nitric acid (HNO₃), K_a = 22

3. bicarbonate (HCO₃⁻), K_a = 6.3 × 10⁻¹¹

4. hydrogen cyanide (HCN), K_a = 7.9 × 10⁻¹⁰

5. formic acid (HCOOH), K_a = 2.0 × 10⁻⁴

6. phosphoric acid (H₃PO₄), K_a = 2.1

For the following acid–base pairs, (iii) predict the favored side of equilibrium; (iv) calculate ;

(f)

Calculate K_eq for these acid–base reactions.

(c)

Given that the indicated pK_a values correspond to the acid dissociation reactions shown, calculate the ratio of acid to conjugate base for the reactions shown.

(b)

Hydrogen gas (H₂) has a relatively high pK_a value. Is it a stable or unstable acid? Do you expect it to participate in acid–base reactions?

For the following acid–base reaction, (b) calculate the equilibrium constant.

For each of the following acid–base reactions, (ii) calculate K_eq. If a pK_a is not one of the ten common ones we learned in Chapter 4, it will be given to you.

(a)