Question

Lactic acid, CH3CH(OH)COOH, is found in sour milk. A solution containing 1.07 gNaCH3CH(OH)COO in 170.0 mL of 0.0500 MCH3CH(OH)COOH has a pH= 3.91.

What is Ka of lactic acid?

Answer 1

number of mole of salt lactic acid = (given mass)/(molar mass)
molar mass of salt lactic acid= 113.1 g/mol
number of mole of lactic acid = 1.07/113.1
= (9.46*10^-3) mole
molarity of salt lactic acid = (number of mole of salt)/(volume of solution)
= (9.46*10^-3)/0.17
= 0.056 M

use,
pH = pKa + log([salt]/[acid])
3.91 = pKa + log(0.056/0.05)
3.91 = pKa + log1.12
3.91 = pKa + 0.049
pKa = 3.86
-log(Ka) = 3.86
log(Ka) = -3.86
Ka = 10^-3.86
Ka = 1.38*10^-4

Answer : 1.38*10^-4