Ka

Question 1

What is the strength of an acid a measure of?

Answer 1

The strength of an acid HA is the extent of its dissociation into H* and A- ions.

Question 2

What is a strong acid?

Answer 2

A strong acid is an acid that completely dissociates in solution.

Question 3

What is Ka?

Answer 3

The actual extent of acid dissociation is measured by an equilibrium constant called the acid dissociation constant Ka.

Question 4

Define the acid dissociation constant.

Answer 4

The acid dissociation constant,Ka, of an acid HA is defined as:

Ka=[H*(aq)][A-(aq)/[HA(aq)]

Question 5

What are the units of Ka?

Answer 5

The units of Ka are always mol dm^-3.

Question 6

What does a large value of Ka show?

Answer 6

A large Ka value indicates a large extent of dissociation - the acid is strong.

Question 7

What is the reason to have to pKa?

Answer 7

Values of Ka can be made more manageable if expressed in a logarithmic form called pKa.

Question 8

Mathematically : pKa =

Answer 8

pKa = -log Ka

Ka = 10-pKa

Question 9

What is the Ka expression for a weak acid [HA] explain how its derived.

Answer 9

Ka = [H(aq)]^2/[HA(aq)]-[H(aq)]

When HA molecules dissociate, [H*] and [A-] ions are formed in equal quantities.

Because relatively few HA molecules have dissociated, [HA] will have reduced slightly. The equilibrium concentration of HA(aq) will be [HA]-[H*].

Question 10

What is the Ka expression for a weak acid [HA] when making an approximation.

Answer 10

Only a very small proportion of HA dissociates. We can assume that the equilibrium concentration of HA will be very nearly the same as the concentration of undissociated HA(aq).

The Ka expression approximates to Ka = [H*(aq)]^2/[HA(aq)]