Mole Concept & Molar Masses — NEET Chemistry MCQs with Solutions

Solution: The mole is the SI unit of amount of substance; one mole contains exactly 6.02214076 × 10²³ elementary entities, usually written 6.022 × 10²³. This fixed value is the Avogadro constant Nₐ (in mol⁻¹). 6.022 × 10²⁴ is ten times too large, and 3.011 × 10²³ is only half of Nₐ.

Solution: Number of moles n = mass ÷ molar mass = 36 g ÷ 18 g mol⁻¹ = 2 mol. Inverting the ratio (18 ÷ 36) gives 0.5 mol; assuming mass equals molar mass gives 1 mol.

Solution: Number of molecules = moles × Nₐ = 2 × 6.022 × 10²³ = 12.044 × 10²³ = 1.2044 × 10²⁴. Forgetting to multiply by the 2 mol leaves just 6.022 × 10²³; a power-of-ten slip gives 1.2044 × 10²³.

Solution: Mass = moles × molar mass = 0.25 × 100 = 25 g. Dividing instead of multiplying (100 ÷ 0.25) gives 400 g; inverting to 0.25 ÷ ... slips give 4 g; using 1 mol gives 100 g.

Solution: 0.5 mol O₂ contains 0.5 × 6.022 × 10²³ = 3.011 × 10²³ molecules. Each O₂ molecule has 2 oxygen atoms, so the number of atoms = 2 × 3.011 × 10²³ = 6.022 × 10²³. Forgetting to double for the two atoms per molecule leaves 3.011 × 10²³.

Solution: Volume at STP = moles × molar volume = 2 × 22.7 = 45.4 L. Using only 1 mol gives 22.7 L; halving gives 11.35 L; using the older 22.4 L mol⁻¹ value (2 × 22.4) gives 44.8 L, whereas current NCERT uses 22.7 L mol⁻¹.

Solution: Moles = number of molecules ÷ Nₐ = 3.011 × 10²³ ÷ 6.022 × 10²³ = 0.5 mol. Treating the count as one full Nₐ gives 1 mol; multiplying instead of dividing distorts it to 2 mol.