Question

You have 1 M solutions of formic acid and sodium formate. Calculate the volume of sodium formate required to make 1 liter of a solution with a pH of 4.1 in which the concentration of formic acid is 0.15 M. The pKa of formic acid is 3.75.

Answer 1

Henderson equation for calculating pH of acidic buffer is given by

pH = pKa + log [salt]/[acid]

Givn that, pH = 4.1, Pka =3.75, [salt] = 1M, [acid] = 0.15M

4.1 = 3.75 + log[salt]/[acid]

0.35 = log[salt]/[acid]

[salt]/[acid] = 10^0.35 = 2.238

[salt] = [acid]*2.238 = 0.15*2.238 = 0.336 M

4.1 = 3.75 + log(0.336*V1)/(0.15*V2)

Solving for V = V1+V2 = 1000mL

To get pH = 4.1, Volume of 0.336 M of sodium formate = 550mL

Volume of 0.15 M of formic acid = 450mL

Intial concn. of Sodium formate = 1M

Then M1V1=M2V2 = 1M*V1 = 0.336*550 = 184.8 mL

Thus, 184.8 mL of 1M sodium formate is required to prepare a buffer with pH 4.1