Guides to molar mass and the mole

Short, plain guides that take you from "what is a mole?" to working out masses from a chemical equation.

The mole is the idea that most of school chemistry rests on, and it is also where many students first feel lost. These pages take it one step at a time. Each guide explains one idea, shows worked examples with every line of working, and points out the usual mistakes. The numbers in the examples are calculated by the same program as the calculator, so they always agree with it.

If you are new to the topic, read them in the order below. If you are revising, go straight to the page you need and finish with the practice questions.

• What is molar mass? Start here
• How to calculate molar mass, step by step Method
• The mole and the Avogadro constant Ideas
• Grams to moles and moles to grams Method
• Percentage composition by mass Method
• Empirical and molecular formula Method
• Hydrates and water of crystallisation Ideas
• Molarity and making up solutions Method
• Molar mass of gases: volume and density Ideas
• Atomic weights: where the numbers come from Ideas
• Molar mass, molecular mass and formula mass Ideas
• Mass calculations from chemical equations Method
• Common mistakes in molar mass sums Practice
• Practice questions with answers Practice

Who these are for

The guides suit students of classes 9 to 12 in India (CBSE, ICSE and state boards), first-year college students, and anyone taking GCSE, A level, IB or a first university chemistry course. They use Indian school spellings such as sulphur, with the other spelling noted where it helps. They are our own writing and are not taken from any textbook.

Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.