The acid dissociation constant, Ka

Question 1

What is an acid dissociation constant, Ka?

Answer 1

It is used to distinguish strong acids from weak acids, where strong acids have a high Ka.

Question 2

What is the general for the dissociation of any weak acid?

Answer 2

HA (aq) = H+ (aq) + A- (aq).

Question 3

What is the calculation for the acid dissociation constant, Ka? Units? Example?

Answer 3

Ka = ([H+] x [A-]) / [HA]

Ka = [H+] x [CH3COO-] / [CH3COOH].

The units are moldm-3

Question 4

What is the usual temperature associated with Ka?

Answer 4

25C.

Question 5

What is the relationship between equilibrium and Ka and dissociation and Ka?

Answer 5

• The larger the Ka value, the further the equilibrium is to the right.
• The larger the Ka value, the greater the dissociation and so the greater the acid strength.

Question 6

What is the limitation with Ka values?

Answer 6

It is too difficult to compare numbers with negative indices.

Question 7

How has the problem with Ka been resolved?

Answer 7

By converting the Ka value into a negative logarithm called pKa:
- pKa = -logKa.
- Ka = 10^-pKa.

Question 8

What is the pKa value of a weak acid with a Ka value of 1.48 x 10^-4 moldm-3?

Answer 8

pKa = -logKa.

pKa = -log (1.48 x 10^-4).

3.38.

Question 9

What is the Ka value of a weak acid with a pKa of 4.82?

Answer 9

Ka = 10^-pKa.

[H+ (aq)] = 10^-4.82.

1.51 x 10^-5.

Question 10

What is the relationship between the acid strength, Ka, and pKa value?

Answer 10

• The stronger the acid, the larger the Ka value and the smaller the pKa value.
• The weaker the acid, the smaller the Ka value and the larger the pKa value.