Calculate the pH of the solution that results when 0.40 g of KHP is dissolved in enough water to produce 25.0 mL of solution.

Question

Potassium hydrogen phthalate (KHP, molar mass = 204.2 g/mol) is a monoprotic weak

acid with K = 3.91 × 10 . Calculate the pH of the solution that results when 0.40 g of

KHP is dissolved in enough water to produce 25.0 mL of solution.

The answer is 3.26

Answer 1

0.40 g KHP x 1 mol/204.2 g = 1.95x10-3 moles/0.025 L = 0.07835 M KHP

KHP ==> K⁺ + HP^- ==> H⁺ + P^2-

Ka = [H+][P2-]/[HP-]

Taking the Ka as 3.91x10^-6 we have..

3.91x10^-6 = (x)(x)/0.07835 -x and assuming x is small we can ignore it in the denominator

x2 = 3.06x10-7

x = 5.53x10^-4 M = [H+] and this is less than 1% of 0.07835 so above assumption was ok.

pH = -log [H⁺] = -log 5.53x10^-4

pH = 3.26