Question

Calculate the pH of a 0.500M formic acid (HCOOH) solution. K_a for formic acid is 1.8×10^-4.

Answer 1

HCOOH ----------> HCOO^-   + H⁺

0.5                            -         -

0.5-X                       X            X

Ka for the reaction = 1.8 *10^-4

Ka expression is given as ,

Substituting the values of Ka and concentration in terms of X,

1.8 *10^-4 * (0.5-X) = X²

1.8 *10^-4 * 0.5 - 1.8 *10^-4 *X =X²

X² + 1.8 *10^-4 *X - 1.8 *10^-4 * 0.5 =0

On solving the above quadratic equation ,we get x= 0.009397, -0.00957726

Since the value of concentration cannot be negative we ill discard the negative root

Hence, x=[H+] = 0.009397

pH is related to [H+] as follows

pH = - Log([H+] )   = - Log( 0.009397 ) = 2.027

Hence pH of the 0.5 M solution of formic acid is 2.027