Question

A.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.

B. Using the Henderson-Hasselbalch equation, explain why the pH of a solution of a weak acid and its conjugate base does not change upon dilution.

Answer 1

A)

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

4.1 = 3.75 + log[Salt]/(0.15)

[salt] = concentration of sodium formate = 0.3358 mol/L

we have the formula M1V1 = M2V2

1 * V1 = 0.3358 * 1

V1 = 0.3358 L of 1M sodium acetate is required to produce 1L of sodium formate solution.

B)

from the pH = pKa + log [salt] / [Acid]

the salt and acid concentration are present in ratio form. so on dilution the changed volume is applied to both concentration as below.

let us assume it is diluted to 2 L, so the concentrations are changed and they represented in the formula.

pH = pKa + log (( [salt] / 2 L) / ( [Acid] / 2 L))

hence it becomes pH = pKa + log ( [salt] / [acid] )

so Ph of the solution does not change upon dilution