Acids and Bases: Weak Acids and Bases Ka for Weak Acids

Question 1

Give the equilibrium for the dissociation of a weak aqueous acid

Answer 1

• HA (aq) ⇌ H+ (aq) + A‒ (aq)

Question 2

What constant do you use when calculating the pH of a weak acid?

Answer 2

• Ka

- Acid dissociation constant

Question 3

Give the Ka expression

Answer 3

• Ka = [H+] [A-] / [HA]

Question 4

Describe the relationship between [H+] and [A-] in weak acids

Answer 4

• [H+] = [A-]

Question 5

If [H+] and [A-] are equal in weak acids, how can the Ka expression be simplified?

Answer 5

• Ka = [H+] [A-] / [HA] becomes

- Ka = [H+]^2 / [HA]

Question 6

How do you calculate the pH of weak acids?

Answer 6

• Write an expression for Ka for weak acid, substitute A- for ion given in question
• Rearrange equation to [H+] = √Ka x [HA]
• Calculate pH using pH = -log10[H+]

Question 7

What scale makes using Ka values more convenient?

Answer 7

• pKa scale

Question 8

Give the expression for calculating pKa

Answer 8

• pKa = -log10 (Ka)

Question 9

How do you convert pKa to Ka?

Answer 9

• Ka = 10^-pKa

Question 10

How do you find the Ka and pKa of weak acids? May be given concentration of HA and pH

Answer 10

• Possible if concentration and pH is known
• Calculate [H+] using [H+] = 10^-pH
• Calculate Ka using Ka = [H+]^2 / [HA]
• Calculate pKa using pKa = -log10 (Ka)

Question 11

How do you calculate the concentration of weak acids? May be given pH and Ka

Answer 11

• Possible if pH and Ka is known
• Calculate [H+] using [H+] = 10^-pH
• Rearrange Ka = [H]^2 / [HA] to [HA] = [H]^2 / Ka

Question 12

How do you calculate the pH of weak acids after dilution? May be given concentration and volume of HA, volume of water and Ka

Answer 12

• Calculate moles of HA
• Calculate [HA] using new volume with water using c=n/v
• Calculate [H+] using [H+] = √Ka x [HA]
• Calculate pH using pH = -log10 [H+]