Soal IChO Ke-26

THEORETICAL PROBLEM No 2

(7 blue points, 6 red points)

Nitrogen in agricultural materials is often determined by the Kjeldahl method. The method involves a treatment of the sample with hot concentrated sulphuric acid, to convert organically bound nitrogen to ammonium ion. Concentrated sodium hydroxide is then added, and the ammonia formed is distilled into hydrochloric acid of known volume and concentration. The excess hydrochloric acid is then back-titrated with a standard solution of sodium hydroxide, to determine nitrogen in the sample.

a) 0.2515 g of a grain sample was treated with sulphuric acid, sodium hydroxide was then added and the ammonia distilled into 50.00 mL of 0.1010 M hydrochloric acid. The excess acid was back-titrated with 19.30 mL of 0.1050 M sodium hydroxide.

Calculate the concentration of nitrogen in the sample, in percent by mass.

b) Calculate the pH of the solution which is titrated in a), when 0 mL, 9.65 mL, 19.30 mL and 28.95 mL of sodium hydroxide has been added. Disregard any volume change during the reaction of ammonia gas with hydrochloric acid. Ka for ammonium ion is 5.7^.10^-10.

c) Draw the titration curve based on the calculations in b).

d) What is the pH transition range of the indicator which could be used for the back titration.

e) The Kjeldahl method can also be used to determine the molecular weight of amino acids. In a given experiment, the molecular weight of a naturally occuring amino acid was determined by digesting. 0.2345 g of the pure acid, and distilling the ammonia into 50.00 mL of 0.1010 M hydrochloric acid. A titration volume of 17.50 mL was obtained for the back titration with 0.1050 M sodium hydroxide.

Calculate the molecular weight of the amino acid based on one and two nitrogen groups in the molecule, respectively.

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