Question

You carefully weigh 5.724 g of potato flour into a Kjeldahl flask and add an appropriate reagent to digest the flour. You then carefully distill the material by adding a reagent to the digestion flask and stirring vigorously, then heating while under a closed system. You collect distillate in a collection flask containing a liquid, then titrate with 0.1 N HCl. It takes 11.95 ml to titrate the sample. The volume of titrate for blank titration is 0.1 ml. What was the percent protein in the potato flour if the protein in potato flour is 16% nitrogen?

Answer 1

Hey there!:

In Kjeldahl's method  estimation of amount of nitrogen in a sample, the sample is first digested by using sulphuric acid , which converts the nitrogen in the sample to ammonium sulphate.

The ammonium ions in the digested product are converted to ammonia gas by adding excess of base like NaOH. The ammonia gas thus produced is collected by condensation.

The amount of ammonia produced is estimated by titration with standard solution of acid. We are using 0.1 N HCl in this case.

Volume of 0.1 N HCl used = 11.95 mL

Volume of of 0.1 N HCl used in blank titration =0.1 mL

Therefore,actual volume us of 0.1 N HCl

used by analyte( ammonia solution) = 11.95 mL - 0.1 mL  

= 11.85 mL

0.1 N HCl = 0.1 M HCl , as HCl is a monobasic acid and therefore its molar mass will be equal to its equivalent mass.

Therefore, concentration of HCl solution used = 0.1 M = 0.1 mol L-1

Volume of HCl solution used =11.85 mL =0.01185 L

Nº.of moles of  HCl used =  0.1 mol L-1  * 0.01185 L

= 0.001185 moles

The equation for reaction between NH3 (aq) and HCl during titration is  :

NH3 (aq) + HCl (aq) <= >  NH4Cl

From the above equation, we see that each mole of HCl reacts completely with 1 mole of NH3 during titration.

Therefore, no. of moles of NH3 that would have been neutralized by 0.001185 moles of HCl = 0.001185 moles

From the formula of  NH3 , 1 mole of  NH3 contains 1 mole of N atoms.

Therefore, nº of moles of :

N - atoms present = 0.001185 moles

Moar mass of N= 14 g/mol

Mass of 0.001185 moles of N =0.001185 moles * 14 g/ mol

=  0.01659 g

% of N in protein is given as 16%.

16%.of mass of protein  = mass of nitrogen in protein sample  = 0.01659 g

(16/100) *mass of protein  =  0.01659 g

mass of protein  =  0.1036875 g

% of protein  in potato flour = [mass of protein in  potato flour / mass of  potato flour] * 100%

( 0.1036875 g / 5.724 ) * 100%

=  1.812 %

Hope this helps!