Percentage Composition of H and O – JEE Main PYQs | Combustion Analysis

On combustion 0.210 g of an organic compound containing C, H and O gave 0.127 g H₂O and 0.307 g CO₂. The percentages of hydrogen and oxygen in the given organic compound respectively are:

A. 7.55, 43.85
B. 6.72, 53.41
C. 6.72, 39.87
D. 53.41, 6.72

Step 1: Calculate moles of CO₂ formed

Moles of CO₂ = 0.307 / 44 = 0.00698 moles

Step 2: Calculate mass of carbon

Moles of carbon = 0.00698 moles
Mass of carbon = 0.00698 × 12 = 0.0838 g

Step 3: Calculate moles of H₂O formed

Moles of H₂O = 0.127 / 18 = 0.00705 moles

Step 4: Calculate mass of hydrogen

Moles of hydrogen = 2 × 0.00705 = 0.0141 moles
Mass of hydrogen = 0.0141 × 1 = 0.0141 g

Step 5: Calculate mass of oxygen

Mass of oxygen = 0.210 − (0.0838 + 0.0141)
= 0.1121 g

Step 6: Calculate percentage of hydrogen

Percentage of hydrogen = (0.0141 / 0.210) × 100
= 6.72%

Step 7: Calculate percentage of oxygen

Percentage of oxygen = (0.1121 / 0.210) × 100
= 53.41%

✅ Final Answer: 6.72%, 53.41% (Option B)

Updated for JEE Main 2026: This PYQ is important for JEE Mains, JEE Advanced and other competitive exams. Practice more questions from this chapter.

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