Percentage composition by mass
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 5 worked examples, 7 practice questions, 1 real past-paper question with the answer explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- Which fertiliser gives more nitrogen?
- What is percentage composition?
- The formula
- The method
Read the transcript of the teaching part
Last time
Before we start: last time, in Empirical formulae and water of crystallisation, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Which fertiliser gives more nitrogen?
A farmer has two bags of fertiliser, and both contain nitrogen. But which bag gives more nitrogen for every kilogram? The answer is in the percentage of nitrogen by mass, and it is worked out from the formula alone.
What is percentage composition?
The percentage by mass of an element in a compound is the part of the total mass that is that element. Every sample of a pure compound has the same percentages. In water, the oxygen is always the same fraction of the mass. The percentages of all the elements in a compound add up to one hundred.
The formula
To find the percentage of an element, take the mass of that element in one formula unit, which is the number of atoms times the Ar. Divide it by the Mr of the whole compound, and multiply by one hundred. Work out the Mr first. Then the arithmetic is easy.
The method
First, work out the Mr of the compound. Second, find the mass of the element in the formula: the number of atoms times the Ar. Third, divide the element's mass by the Mr. Fourth, multiply by one hundred, and give the answer with a percent sign.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Percentage by mass.
Chapter 3: Stoichiometry
- Relative formula mass 6 min
- Formulae of ionic compounds 6 min
- Writing and balancing equations 7 min
- Ionic equations 7 min
- The mole and the Avogadro constant 6 min
- Moles: amount, mass and molar mass 6 min
- Reacting masses 7 min
- Gas volumes at r.t.p. 7 min
- Concentration and titration calculations 9 min
- Percentage composition by mass 6 min
- Empirical and molecular formulae 7 min
- Water of crystallisation 7 min
Also for IGCSE Chemistry 0620
Which topics come up most in IGCSE Chemistry 0620 past papers, counted from every paper since 2004.