A gas cylinder contains 12 x 1024 molecules of oxygen gas. If Avogadro's number is 6 x 1023; Calculate:
the mass of oxygen present in the cylinder.
Topic: Mole concept, gas volumes and stoichiometry
Answer
Given: 12 × 10²⁴ molecules of O₂, Avogadro's number = 6 × 10²³.
Moles of O₂ = number of molecules ÷ Avogadro's number = (12 × 10²⁴) ÷ (6 × 10²³) = 20 mol
Molar mass of O₂ = 2 × 16 = 32 g mol⁻¹
Mass = moles × molar mass = 20 × 32 = 640 g
Mass of oxygen = 640 g
Everything in the mole chapter passes through moles. Molecules divide by Avogadro's number to give moles; moles multiply by molar mass to give grams.
The commonest error is using 16 instead of 32. Oxygen GAS is O₂ — a molecule of two atoms — so its molar mass is 32, not 16. Using 16 halves the answer to 320 g. Watch the powers too: 10²⁴ ÷ 10²³ is 10, so 12 ÷ 6 × 10 = 20, not 2.