COMMON ION EFFECT

Question

1.) What is the pH of a solution that is 0.30M in HCOOH and 0.52M in HCOOK.
Ka of HCOOH=1.7x10^-4.
2.) Calculate the pH of a buffer system containing 1.0M CH3COOH and 1.0M CH3COONa. Ka of CH3COOH=1.8x10^-5.

Answer 1

1)
I notice that your value for Ka in the question is missing units, were these missing in the original question?

Start from the equation for the equilibrium reaction

HCOOH<->H+ + HCOO-

at start
0.30M

at equilibrium if [H+]=x

0.30-x x x

there is also 0.52M HCOO-

so at equilibrium

0.30-x x x+0.52

Ka=[H+][HCOO-]/[HCOOH]

Ka=(x)(x+0.52)/(0.30-x)=1.7x10^-4

(x)(x+0.52)/(0.30-x)=1.7x10^-4

You can either solve the quadratic or we can say that if x is small with respect 0.52 abd 0.3, we can rewrite the expression as

(x)(0.52)/(0.30)=1.7x10^-4

and find x

pH is then -log (x/mol litre^-1)