Question

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 where CP = 6.38 (that is % protein = CP x %N)

**PLEASE SHOW ME STEP BY STEP CALCULATION**

Answer 1

mass of sample = 100 g

volume HCl = 100 mL = 0.100 L

molarity HCl = 0.5 M

total moles HCl = (molarity HCl) * (volume HCl in Liter)

total moles HCl = (0.5 M) * (0.100 L)

total moles HCl = 0.05 mol

volume NaOH = 34.50 mL = 0.03450 L

molarity NaOH = 0.3512 M

moles NaOH = (molarity NaOH) * (volume NaOH in Liter)

moles NaOH = (0.3512 M) * (0.03450 L)

moles NaOH = 0.012 mol

moles HCl consumed by NaOH = moles NaOH

moles HCl consumed by NaOH = 0.012 mol

moles HCl consumed by ammonia = total moles HCl - moles HCl consumed by NaOH

moles HCl consumed by ammonia = 0.05 mol - 0.012 mol

moles HCl consumed by ammonia = 0.038 mol

moles ammonia present = moles HCl consumed by ammonia

moles ammonia present = 0.038 mol

moles Nitrogen present = moles ammonia present

moles Nitrogen present = 0.038 mol

mass nitrogen = (moles nitrogen) * (molar mass nitrogen)

mass nitrogen = (0.038 mol) * (14.0 g/mol)

mass nitrogen = 0.530 g

% N = (mass nitrogen / mass milk sample) * 100

% N = (0.530 g / 100 g) * 100

% N = 0.530 %

% protein = CP * % N

% protein = (6.38) * (0.530 %)

% protein = 3.38 %