Chemistry 11 · Chemistry · Lesson 4.2
The mole and molar mass
- Level
- Grade 11
- Reading time
- 11 min reading
- Review
- Last reviewed 2026-09-03
- Standard
- Accuracy standard
- Errata
Abstract
Defining the mole and finding the molar mass of a compound.
What is molar mass in chemistry?
Short answer
Molar mass is the mass of one mole of a substance, in grams per mole (g/mol). For an element it equals the atomic mass on the periodic table; for a compound you add the molar masses of every atom in the formula. Molar mass is the bridge that converts between the mass of a sample and the number of moles it contains.
Concept preview
What you will be able to do
- 01Explain why chemists count particles by weighing
- 02State the value of a mole and of Avogadro's number
- 03Calculate the molar mass of an element, compound or hydrate
To make iron(II) sulfide, , you need equal numbers of iron and sulfur atoms, but atoms are far too small to count. One iron atom has a mass of about — impossible to weigh individually. The way out is to weigh amounts that contain equal numbers. Because the periodic table tells you that iron atoms are times as heavy as sulfur atoms, weighing out of iron and of sulfur guarantees equal numbers of each. Chemists count by weighing.
School Plan
The mole and molar mass
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Related topics
Frequently asked questions
- How do you calculate molar mass?
- Add the molar mass of each atom in the chemical formula. For H₂O: 2 × 1.01 + 16.00 = 18.02 g/mol.
- What is the difference between molar mass and molecular mass?
- They are numerically equal; molar mass is expressed in g/mol (per mole), molecular mass in atomic mass units (per molecule).