Some Basic Concepts of Chemistry for NEET UG
The mole is chemistry's counting unit. Once you can convert between mass, moles and particles, and balance equations, every quantitative chemistry problem becomes manageable.
ChemistryClass 1174 MCQs3 lessons12 flashcards4 memory tricks
Practise Some Basic Concepts of Chemistry free
What you'll learn
- The mole and molar mass
Convert between mass, moles, number of particles and gas volume. - Empirical and molecular formulas
Find the empirical formula from percentage composition, and the molecular formula from molar mass. - Stoichiometry and the limiting reagent
Use a balanced equation to find amounts of products and identify the limiting reagent.
Key formulas
| Moles from mass | n = mass/molar mass |
|---|---|
| Particles | N = n × N_A |
| Gas volume at STP (273 K, 1 atm) | V = n × 22.4 L |
| Molarity | M = moles of solute / volume of solution (L) |
| Molality | m = moles of solute / mass of solvent (kg) |
| Mole fraction | x_A = n_A/(n_A + n_B) |
Memory tricks
The limiting reagent is used up first and decides how much product forms. Divide the moles of each reactant by its coefficient; the smallest answer is the limiting one.
Glucose: empirical CH₂O, molecular C₆H₁₂O₆. Molecular formula = n × empirical formula, where n = molar mass ÷ empirical-formula mass.
Molarity = mol per litre of solution, which changes with temperature. Molality = mol per kg of solvent, which does not.
1 mol = 6.022 × 10²³ particles, and one mole of a gas at STP occupies 22.4 L (22.7 L at the 1 bar standard).
Try 3 NEET-style questions
Q1. The mass of water produced by the complete combustion of 16 g of methane (CH₄ + 2O₂ → CO₂ + 2H₂O) is
- 36 g
- 18 g
- 44 g
- 16 g
Show answer
Answer: (1) 36 g
16 g of CH₄ is 1 mol. It produces 2 mol of H₂O, which is 2 × 18 = 36 g.
Q2. The mass of one atom of carbon-12 is about (N_A = 6.022 × 10²³ mol⁻¹)
- 1.99 × 10⁻²³ g
- 12 g
- 7.2 × 10²⁴ g
- 1.66 × 10⁻²⁴ g
Show answer
Answer: (1) 1.99 × 10⁻²³ g
Mass of one atom = 12 g / 6.022 × 10²³ ≈ 1.99 × 10⁻²³ g.
Q3. The molar mass of water (H₂O) is
- 16 g/mol
- 18 g/mol
- 20 g/mol
- 2 g/mol
Show answer
Answer: (2) 18 g/mol
M(H₂O) = 2 × 1 + 16 = 18 g/mol.
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