Reacting masses
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 6 worked examples, 7 practice questions, 2 real past-paper questions with the answers explained, the mistakes that lose marks, the full-marks tip, a one-card summary and a quick quiz.
What this lecture covers
- How much will I make?
- What the equation tells you
- The method
Read the transcript of the teaching part
Last time
Before we start: last time, in Percentage composition by mass, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
How much will I make?
A factory wants to make a tonne of a product. How much of the raw material must it buy? Too little, and the order is not filled. Too much, and money is wasted. The balanced equation and the mole give the answer before anyone opens a bag.
What the equation tells you
The coefficients in a balanced equation give the ratio of moles that react and form. In magnesium plus oxygen, two Mg plus O₂ gives two MgO, two moles of magnesium make two moles of magnesium oxide. It is moles that react in these ratios. It is not the masses.
The method
First, write the balanced equation, and underline the two substances in the question. Second, find the moles of the substance you know: moles equals mass over Mr. Third, use the equation to find the moles of the substance you want. Fourth, find the mass of that substance: mass equals moles times Mr. Fifth, check the units, and give the answer with the correct number of significant figures.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Moles: reacting masses.
The whole chapter: Stoichiometry, Stoichiometry.
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.