Percent ionization for a weak acid (HA) is determined by the following formula:

Question

Percent ionization for a weak acid (HA) is determined by the following formula:

Percent ionization=[HA] ionized[HA] initial×100%

For strong acids, ionization is nearly complete (100%) at most concentrations. However, for weak acids, the percent ionization changes significantly with concentration. The more diluted the acid is, the greater percent ionization.

A certain weak acid, HA, has a Ka value of 9.4×10?7.

Part A

Calculate the percent ionization of HA in a 0.10 M solution.

Part B

Calculate the percent ionization of HA in a 0.010 M solution.

Explanation / Answer

The weak acid will dissciate as

                   HA ---> H+ + A-

Initial            a      0     0
Change        -x          +x   +x

Equilibri     a-x       x      x

Ka = [H+][A-]/[HA] = x2 / (a-x) = 9.4×10?7

as Ka is very low we may ignore x in denominator

9.4×10?7 = x2 / (a)

x = (Ka X concentration)1/2

Where x is amount dissciated

(i) for a = 0.10

x = (0.10 X 9.4×10?7)1/2 = 3.066 X 10-4

Percentage of dissociation = x . 100 / a = 3.066 X 10-4 X 100 / 0.1 = 0.3066 %

(ii) for a = 0.010 M

x = (0.010 X 9.4×10?7)1/2 = 0.969 X 10-4

Percentage of dissociation = x . 100 / a = 0.969 X 10-4 X 100 / 0.01 = 0.969 %

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