Calculate the pH of a solution that contains 0.250 M formic acid, HCOOH (Ka =1.8 x 10-4 ), and 0.100M sodium formate, HCOONa after the addition of 10.0 mL of 6.00M NaOH to the original buffered solution volume of 500.0 mL.

Answer : The pH of the solution is 3.5

Explanation :

First we have to calculate the moles of , and .

and,

and,

The equilibrium chemical reaction is:

Initial moles      0.125          0.06      0.05

At eqm.          (0.125-0.06)     0      (0.05+0.06)

                        = 0.185                        = 0.11

Given:

Now we have to calculate the value of .

The expression used for the calculation of is,

Now put the value of in this expression, we get:

Now we have to calculate the pH of buffer.

Using Henderson Hesselbach equation :

As the volume of solution are same. So,

Now put all the given values in this expression, we get:

Therefore, the pH of the solution is 3.5