You want to determine the protein content in milk with the Kjeldahl method. You take 100 g whole milk and use 100 mL of 0.5 M hydrochloric acid to collect ammonia. You needed 34.50 mL of 0.3512 M NaOH for the back-titration. Calculate the percentage of protein in the sample?

82.53 % protein in the milk.

Explanation:

Titration formula:

Conc (acid) × Vol (acid) = Conc (base) × Vol (base)

Beginning from back-titration:

0.5 M HCl × Vol  (HCl) = 0.3512 (conc of NaOH) × 34.50 mL (vol of NaOH)

=   = 24.2328 mL

⇒ Vol of HCl used in initial titration is 24.2328 mL

So from Initial Titration:

Using same titration formula,

0.5 × 24.2328 = Conc. (base) × 100 ml

Conc (base) = 12.1164 ÷ 100

= 0.121164 M.

But concentration = mass ÷ molar mass

0.121164 =

= 825.3277 g/mol of protein

⇒ × 100

= 82.53 % protein in the milk.