All question types · Stoichiometry

Number of atoms in a mass of compound

A-Level (AQA) · unreviewed

Mass of a molecular compound; count the atoms of one element in it.

Practise this with fresh numbers

Worked example

Calculate the number of hydrogen atoms in 12.0 g of ethane, C₂H₆ (M = 30.0 g mol⁻¹). Use the Avogadro constant, 6.022 × 10²³ mol⁻¹.

1. Moles from mass: solve for n(C₂H₆)
$n = \dfrac{m}{M}$
= 0.4000 mol
2. Number of particles: solve for Molecules
$N = nN_\mathrm{A}$
= 2.409 × 10²³
3. Multiply by a coefficient: 6 × Molecules → H atoms (6 per molecule)
= 1.445 × 10²⁴

Answer: Number of H atoms = 1.45 × 10²⁴

Method

n = m/M, then molecules = n × N_A. Each ethane molecule has six H atoms, so multiply by 6.

Common mistakes

That is the number of molecules. Each C₂H₆ molecule contains six hydrogen atoms.

Formulas used

Moles from mass
$n = \dfrac{m}{M}$
Number of particles
$N = nN_\mathrm{A}$

Practise this with fresh numbers

More stoichiometry questions