Chemistry MCQs Review

Overview

This lecture reviews solved MCQs from JEE Main 2025 (Evening Shifts) focusing on "Some Basic Concepts of Chemistry," including combustion analysis, gas laws, solution stoichiometry, and estimation methods.

Combustion Analysis and Elemental Composition

• Combustion of a 0.210 g organic compound (C, H, O) produced 0.127 g H₂O and 0.307 g CO₂.
• The question required calculation of percentage composition of hydrogen and oxygen in the compound.

Gas Volumes and Moles at STP

• Calculation of magnesium mass needed to produce 220 mL hydrogen gas at STP using its reaction with dilute HCl.
• Given: molar mass of Mg = 24 g/mol.

Solution Mixing and Concentration

• 10 mL of 2 M NaOH mixed with 20 mL of 1 M HCl; then 10 mL of the mixture diluted in 100 mL flask with 2 moles HCl.
• The resulting solution's composition (NaCl, HCl, or neutrality) was assessed.

Dumas' Method and Nitrogen Estimation

• 0.4 g of an organic compound gave 60 mL of N₂ at 300 K and 715 mm Hg in Dumas' method.
• The calculation of nitrogen content by correcting for aqueous tension (given as 15 mm Hg).

Key Terms & Definitions

• Combustion Analysis — Analytical method to determine elemental composition by measuring masses of CO₂ and H₂O produced.
• STP (Standard Temperature and Pressure) — Conditions of 0°C (273 K) and 1 atm, used for gas volume calculations.
• Dumas' Method — A technique for estimating nitrogen in organic compounds by measuring evolved N₂ gas.
• Aqueous Tension — The vapor pressure of water at a given temperature, subtracted from gas pressure readings.

Action Items / Next Steps

• Practice similar numerical problems on combustion analysis and gas calculations.
• Review stoichiometry for solution mixing and limiting reagent concepts.
• Understand correction of gas volumes for aqueous tension in practical estimation methods.