The mole and the Avogadro constant
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The teaching part (1 min) is free to watch. A free account opens the whole lecture: 5 worked examples, 6 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
- Counting the uncountable
- What is a mole?
- Why that number?
- Counting particles
Read the transcript of the teaching part
Last time
Before we start: last time, in Reacting masses and gas volumes, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.
Counting the uncountable
A single drop of water contains billions upon billions of molecules. Nobody can count them one by one. So chemists count in a huge unit, the way a shop counts eggs by the dozen. That unit is the mole.
What is a mole?
One mole of any substance contains the same number of particles: six point oh two times ten to the twenty-three. This number is called the Avogadro constant. It is written as NA, or just six point oh two times ten to the twenty-three per mole. The particles can be atoms, molecules, ions or electrons. You must say which.
Why that number?
The number is chosen so that one mole of a substance has a mass in grams equal to its relative formula mass. So one mole of carbon atoms is twelve grams, and one mole of water molecules is eighteen grams. Both contain the same number of particles, six point oh two times ten to the twenty-three. We will use this idea in the next lesson.
Counting particles
To find the number of particles, multiply the number of moles by the Avogadro constant. To go the other way and find the moles, divide the number of particles by the Avogadro constant. Always give the answer in standard form, and say what the particles are.
How often it comes up
How often Cambridge has asked each part, counted by ExamYard from every past paper in its question bank: Moles: the mole and the Avogadro constant · The Avogadro constant.
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.