An organic compound contains C , H , N and O . A 1.50 g sample of this compound on combustion gives 2.64 g of CO 2 and 0.63 g of H 2 O . In a separate experiment, 1.50 g of the same compound is an

Question

An organic compound contains C , H , N and O . A 1.50 g sample of this compound on combustion gives 2.64 g of CO 2 ​ and 0.63 g of H 2 ​ O . In a separate experiment, 1.50 g of the same compound is analyzed by Kjeldahl's method and the ammonia evolved neutralized 30 mL of 0.25 MH 2 ​ SO 4 ​ . Find the mass fraction of oxygen in the compound.

TestHub · Accepted Answer

Explain the question: Determine the mass fraction of oxygen in an organic compound.

Concept: Principle of Conservation of Mass.

Solution:

(1) Calculate the mass of Carbon (C):

From 2.64 g of CO₂ (molar mass = 44 g/mol): Mass of C = $\frac{2.64 g}{44 g / m o l} \times 12 g / m o l = 0.72 g$

(2) Calculate the mass of Hydrogen (H):

From 0.63 g of H₂O (molar mass = 18 g/mol):

Mass of H = $\frac{0.63 g}{18 g / m o l} \times 2 g / m o l = 0.07 g$

(3) Calculate the mass of Nitrogen (N) from titration:

The ammonia (NH₃) evolved from 1.50 g of the compound neutralized 30 mL of 0.25 M H₂SO₄.

Moles of H₂SO₄ = $0.25 m o l / L \times 0.030 L = 0.0075 m o l$

From the reaction $2 N H_{3} + H_{2} S O_{4} \to (N H_{4})_{2} S O_{4}$ , moles of NH₃ (and thus N) = $2 \times 0.0075 = 0.015 m o l$ .

Mass of N = $0.015 m o l \times 14 g / m o l = 0.21 g$

(4) Calculate the mass of Oxygen (O):

Mass of O = Total mass – (Mass of C + Mass of H + Mass of N)

Mass of O = $1.50 g - (0.72 g + 0.07 g + 0.21 g) = 1.50 g -1.00 g = 0.50 g$

(5) Calculate the mass fraction of Oxygen: Mass fraction = $\frac{M a sso f O}{T o t a l ma ss} = \frac{0.50}{1.50} = \frac{1}{3} \approx 0.333$

The calculated value of 0.333 is closest to option (c).

Final Answer: Option (c)

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