Stoichiometry — Part 1: Percentage composition and formulae: visual lesson, ICSE Class 10 Chemistry (Science Paper 2)

Lesson

What is a compound made of?

Part 1 — a formula tells us how many atoms of each element are in a compound. Using atomic masses, we can turn a formula into percentages by mass, and percentages back into a formula.

In this lesson you will

Calculate the percentage by mass of each element, and of water of crystallisation, in a compound
Find the empirical formula of a compound from its percentage composition
Find the molecular formula from the empirical formula and the vapour density or molar mass
Find an empirical formula from the masses of elements that combine

Table of four compounds with their formulae and molar masses
Compound	Formula	Molar mass / g
Ammonium nitrate	NH₄NO₃	80
Calcium carbonate	CaCO₃	100
Glucose	C₆H₁₂O₆	180
Hydrated copper(II) sulphate	CuSO₄·5H₂O	250 (Cu = 64)

Concept

Percentage composition

Percentage composition gives the percentage by mass of each element in a compound.

% of an element = (mass of that element in 1 mol of the compound ÷ molar mass) × 100

For urea, CO(NH₂)₂, M = 12 + 16 + 2 × 14 + 4 × 1 = 60, so nitrogen = 28 ÷ 60 × 100 = 46.67%. The percentages of all the elements add up to 100.

Worked example

Worked example: nitrogen in ammonium nitrate

Problem. Calculate the percentage of nitrogen in ammonium nitrate, NH₄NO₃. (N = 14, H = 1, O = 16)

Step 1. Molar mass $= 2 \times 14 + 4 \times 1 + 3 \times 16 = 80$ g.

Step 2. Mass of nitrogen in 1 mol $= 2 \times 14 = 28$ g (one N in NH₄⁺, one in NO₃⁻).

Step 3. Percentage of nitrogen $= \frac{28}{80} \times 100 = 35$ %.

Table of the mass and percentage of nitrogen, hydrogen and oxygen in ammonium nitrate
Element	Mass in 1 mol / g	Percentage
Nitrogen	28	35%
Hydrogen	4	5%
Oxygen	48	60%
Total	80	100%

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