Question

What is the pH of a buffer solution (0.100 L containing 0.166 mol/L pyruvic acid (K_a = 4.1x10^-3)) and 0.117 mol L^-1 sodium pyruvate before and after the addition of 0.010 L of 0.10 mol/L HCl?

Answer 1

This is a buffer so

[Acid] = 0.166

pKa = -log(Ka) = -log(4.1*10^-3 )= 2.3872

then

buffer equation

pH = pKa + log(A-/HA)

pH = 2.3872 + log(0.117/0.166) = 2.2352

after addition of HCl

VT = V1+V2 = 0.1+0.01 = 0.11

mmol of HCLadded = MV = 0.01*0.1 = 0.001 molof HCl

mmol of acid initially = MV = 0.1*0.166 = 0.0166

mmol of conjguate initialy = MV = 0.117*0.1 = 0.0117

then, after addition of acid

mmol of acid formed= 0.0166 + 0.001 = 0.0166

mmol of conjguate reacted 0.0117 - 0.001 = 0.0107

then

pH = 2.3872 + log(0.0107/0.0166) = 2.19647