Question

Lactic acid, HC3H5O3(aq) is a weak acid that gives yougurt its sour taste(Yeeeeecccckkk). Calculate the pH of a 0.0010 mol/L solution of Lactic acid. The Ka for lactic acid is 1.4 x 10^-4

Answer 1

Let's call Lactic acid HL.
HL ==> H^+ + L^-

Ka = (H^+)(L^-)/(HL) = 1.4 x 10^-4

initially before ionization:
(HL) = 0.001 M
(H^+) = 0
(L^-) = 0

change:
(H^+) = +x
)L^-) = +x
(HL) = 0.001 - x

equilibrium:
(H^+) = +x
(L^-) = +x
(HL) = 0.001 - x

Substitute the equilibrium values shown into the Ka expression and solve for x.
Then convert (H^+) to pH by pH = -log(H^+)

Answer 2

2.5