Physics A2 · Ideal gases. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Amount of substance is an SI base quantity. Its base unit is the mole (mol). It counts particles in very large groups, so that we can link the tiny scale of atoms and molecules to masses we can measure.
One mole of any substance is the amount that contains a number of particles equal to the Avogadro constant, NA = 6.02 × 1023 mol−1. The particles may be atoms, molecules or ions, so always check which one the question asks for.
The molar mass is the mass of one mole. With it you can move between mass, amount in moles and number of particles.
Students mix grams with kg mol−1, giving answers 1000 times wrong. Use the same mass unit for the mass and the molar mass. Also count atoms per molecule if the question asks for atoms.
Answer: About 1.5 × 1024 molecules.
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