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NVAEasyJEE 2026Quantitative Analysis (C, H, N…)

JEE Chemistry 2026 Question with Solution

0.25 g of an organic compound “A” containing carbon, hydrogen and oxygen was analysed using combustion method. The increase in mass of CaCl2_2 tube and potash tube at the end of the experiment was found to be 0.15 g and 0.1837 g respectively. The percentage of oxygen in compound A is _____ (Nearest integer).

Answer

Correct answer:73

Step-by-step solution

Standard Method

Given: Mass of compound =0.25 g= 0.25 \, \text{g}. Increase in mass of CaCl2\text{CaCl}_2 tube =0.15 g= 0.15 \, \text{g}. Increase in mass of potash tube =0.1837 g= 0.1837 \, \text{g}.

Find: Percentage of oxygen in compound A.

Step 1: Mass of hydrogen CaCl2\text{CaCl}_2 absorbs water.

Mass of H=218×0.15=0.0167 g\text{Mass of H} = \frac{2}{18} \times 0.15 = 0.0167 \, \text{g}

Step 2: Mass of carbon Potash tube absorbs CO2\text{CO}_2.

Mass of C=1244×0.1837=0.0501 g\text{Mass of C} = \frac{12}{44} \times 0.1837 = 0.0501 \, \text{g}

Step 3: Mass of oxygen

Mass of O=0.25−(0.0167+0.0501)=0.1832 g\text{Mass of O} = 0.25 - (0.0167 + 0.0501) = 0.1832 \, \text{g}

Step 4: Percentage of oxygen

Percentage of O=0.18320.25×100≈73\text{Percentage of O} = \frac{0.1832}{0.25} \times 100 \approx 73

Therefore, the percentage of oxygen in compound A is 73%73\%, so the numerical answer is 73.

Common mistakes

  • Using the full increase in mass of the CaCl2\text{CaCl}_2 tube as mass of hydrogen is incorrect because the tube absorbs H2O\text{H}_2\text{O}, not hydrogen alone. First convert water mass to hydrogen mass using 218\frac{2}{18}.

  • Using the full increase in mass of the potash tube as mass of carbon is incorrect because the potash tube absorbs CO2\text{CO}_2. First convert CO2\text{CO}_2 mass to carbon mass using 1244\frac{12}{44}.

  • Calculating oxygen percentage directly from combustion products is incorrect here because oxygen is found by difference. After finding masses of carbon and hydrogen, subtract them from the original sample mass to get oxygen mass.

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