Define mole and molar volume. Calculate the number of molecules in 36 g of water.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Define mole. One mole is the amount of substance that contains as many particles as there are atoms in 12 g of C-12, that is 6.023 × 10²³ particles.
2 · Define molar volume. One mole of any gas at STP (273 K, 1 atm) occupies 22.4 litres.
3 · Draw the mole bridge. [Mass ÷ molar mass → moles; moles × 6.023 × 10²³ → particles; moles × 22.4 L → gas volume at STP]
4 · Work out the sum.
Molar mass of H₂O = 2(1) + 16 = 18 g mol⁻¹
Moles = 36 ÷ 18 = 2 mol
Molecules = 2 × 6.023 × 10²³ = 1.2046 × 10²⁴
5 · Real-life link. A glass of water (180 g) is 10 moles, about 6 × 10²⁴ molecules.
6 · Close in one line. Therefore 36 g of water contains 1.2046 × 10²⁴ molecules.
Scoring tip: Always find the number of moles first. Write 'molecules' or 'atoms' after the answer, never a bare number.
