Moles: amount, mass and molar mass
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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
- From the balance to the mole
- Molar mass
- The mole formula
- The method
Read the transcript of the teaching part
Last time
Before we start: last time, in Ionic equations, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
From the balance to the mole
In the lab you cannot count particles, but you can weigh a substance on a balance. The mole connects the two. With one formula, you can turn any mass into an amount in moles, and any amount in moles into a mass.
Molar mass
The molar mass of a substance is the mass of one mole of it. Its unit is grams per mole. It has the same number as the relative formula mass, Mr, but now with units. So carbon dioxide has Mr forty-four, and a molar mass of forty-four grams per mole.
The mole formula
The amount in moles, n, equals the mass in grams, divided by the molar mass, which is the Mr. Rearranged, the mass equals the moles multiplied by the Mr. And the Mr equals the mass divided by the moles. Use the units to check. Mass is in grams, amount is in moles, and Mr is in grams per mole.
The method
First, write down the formula and work out the Mr. Second, write down what you are given and what you need to find. Third, use the correct form of the formula, one line at a time, with units. Fourth, check that the answer is sensible, and give the unit.
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
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