Question

Calculate the concentration of the lactate ion in a solution that is 0.100 M in lactic acid (CH3CH(OH)COOH, pKa = 3.86) and 0.080 M in HCl.

	4.83 M
	0.0800 M
	7.3×10−3 M
	3.65×10−3 M
	1.73×10−4 M

Answer 1

CONCEPT: The question isbased on common ion effect. The question can be solved by using ICE equation. There is0.08M H+ ions in the solution before dissociation of lactic acid.

(CH3CH(OH)COOH <---------------> (CH3CH(OH)COO^- + H⁺

initial 0.1 0 0.08

change -x +x +x

Eq 0.1 - x x 0.08 + x

pKa = [(CH3CH(OH)COO^- ][H+]/[(CH3CH(OH)COOH]

pKa = -logKa

Ka = 10^-pKa = 10^3.86    = 1.38 X 10^-4

1.38 X 10^-4 = x * (0.08 + x)/ (0.1 - x)

we get quadraitic eqation

x² + 0.080138x +  1.38 X 10^-5 = 0

solving the equation gives x = 1.72 X 10^-4

Hence concentratio of lactate ion = 1.72 X 10^-4