Question

Find the pH of a 0.150M solution of a weak monoprotic acid having Ka= 0.20.

Answer 1

Solution

Given data

Concentration of acid = 0.150 M

Ka = 0.20

pH= ?

lets say monoprotic acid is HA

now lets write reaction of acid with water and make the ICE table

        HA + H2O -----> H3O⁺ + A^-

I      0.150 M                 0            0

C        -x                      +x            +x

E    0.150-x                  x              x

Now write the ka equation for this reaction

Ka = [H3O⁺ ][ A^-]/[ HA]

Now lets put the values in the formula

0.20 = [x][x]/[0.150-x]

0.20 * [0.150-x] = x²

By solving this using quadratic equation we get

[H3O⁺]=X= 0.1 M

Now using this concentration calculate the pH

pH= -log [H3O⁺]

pH