Question

What is the pH of a buffer solution (100 mL containing 0.50 mol L-1 pyruvic acid (Ka = 4.1x10-3) and 0.35 mol L-1 sodium pyruvate after the addition of 10 mL of 0.10 mol L-1 HCl?

Answer 1

First calculate moles of pyruvic acid , sodium pyruvate and HCl.

We have, Molarity = No. of moles of solute / volume of solution in L

No. of moles of solute = Molarity volume of solution in L

No. of moles of pyruvic acid ( CH₃COCOOH ) = 0.50 mol / L 0.100 L = 0.050 mol

No. of moles of sodium pyruvate (CH₃COCOONa )= 0.35 mol / L 0.100 L = 0.035 mol

No. of moles of HCl = 0.10 mol / L 0.010 L = 0.0010 mol

Consider reaction of HCl with buffer solution.

CH₃COCOONa (aq) + HCl (aq) CH₃COCOOH (aq) + NaCl (aq)

Let's use ICE table.

Moles	CH3COCOONa (aq) + HCl (aq) CH3COCOOH (aq)
I	0.035	0.0010	0.050
C	-0.0010	-0.0010	+0.0010
E	0.034	0.0000	0.051

After addition of HCl, volume of buffer solution is 100 ml + 10 ml = 110 ml = 0.110 L

We can calculate pH of Buffer solution by using Henderson's equation, pH = pKa + log [ Salt ]/ [ acid]

pH = - log Ka + log [ CH₃COCOONa ] / [ CH₃COCOOH ]

pH = - log ( 4.1 10 ^-03 ) + log ( 0.034 mol / 0.110 L ) / ( 0.051 mol / 0.110 L )

pH = 2.39 + log ( 0.034 mol / 0.110 L ) / ( 0.051 mol / 0.110 L )

pH = 2.39 - 0.176

pH = 2.214

ANSWER : pH of pyruvate buffer solution after addition of HCl is 2.21.