Quantitative Analysis - Problem 10

Description:

Problem − A sample of 0.50 g of an organic compound was treated according to Kjeldahls method. The ammonia evolved was absorbed in 50 mL of 0.5 M H₂SO₄. The residual acid required 60 mL of 0.5 M solution of NaOH.

Solution − Given,

Amount of sample = 0.50 g

Volume of H₂SO₄ (0.5 M) taken = 50 mL

Volume of NaOH (0.5 M) required to titrate residual/excess acid = 60 mL.

2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

Volume of H₂SO₄ consumed in the above reaction = 30 mL

Volume of 0.5 M H₂SO₄ used in the reaction with ammonia = 50 − 30 = 20 mL.

2NH₃ + H₂SO₄ → (NH₄)₂SO₄

Millimoles of H₂SO₄ used = 0.5 × 20 = 10 m mol

Millimoles of NH₃ produced = 10×2 = 20 m mol

Mass of NH₃ produced = 20 × 17× 10 ⁻³ = 0.34 g

Mass of nitrogen produced = 0.34 ×

14

/

17

= 0.28

Percentage of nitrogen =

0.28

/

0.5

× 100 = 56%

Alternatively,

Percentage of nitrogen =

1.4 × N_acid × V_acid

/

mass of organic compound

=

1.4 × 0.5 × 2 × 20

/

0.5

= 56%