Molar MassGrams to moles and moles to grams
This is the conversion used in almost every chemistry calculation. One formula does both directions.
moles = mass in grams ÷ molar mass (n = m ÷ M)
mass in grams = moles × molar mass (m = n × M)
Grams to moles
Example 1. How many moles are in 100 g of calcium carbonate, CaCO3? Its molar mass is 100.09 g/mol. Moles = 100 ÷ 100.09 = 0.999 mol, which is almost exactly 1 mole.
Example 2. How many moles are in 4.9 g of sulphuric acid? Moles = 4.9 ÷ 98.07 = 0.0500 mol, about 0.05 mol.
Example 3. A tablet contains 500 mg of aspirin, C9H8O4. First change milligrams to grams: 0.500 g. Moles = 0.500 ÷ 180.16 = 0.00278 mol.
Moles to grams
Example 4. What is the mass of 0.2 mol of sodium hydroxide? Mass = 0.2 × 40.00 = 8.00 g.
Example 5. A reaction needs 1.5 mol of water. Mass = 1.5 × 18.02 = 27.02 g, which is also about 27 mL because 1 mL of water has a mass of about 1 g.
A way to remember
Many students draw a triangle with mass on top and moles and molar mass side by side below. Cover the quantity you want. If the other two are side by side, multiply them. If one is above the other, divide the top by the bottom. Units help too: g divided by g/mol leaves mol.
Kilograms, milligrams and tonnes
The molar mass is in grams per mole, so the mass must be in grams before you divide. 1 kg is 1000 g. 1 mg is 0.001 g. A tonne is 1,000,000 g. Industrial questions about fertiliser or cement often give tonnes; convert first, or work in kilomoles, where 1 kmol of a substance has a mass in kilograms equal to its molar mass number.
Usual mistakes
- Dividing the wrong way. If you have much less than the molar mass in grams, the answer must be less than 1 mole.
- Using the atom instead of the molecule. 32 g of oxygen gas is 1 mole of O2, not 2 moles, unless the question asks for atoms.
- Forgetting the water in a hydrate. 25 g of blue copper sulphate crystals is 25 ÷ 249.68 = 0.100 mol, not 25 ÷ 159.60.
- Rounding too early. Keep four significant figures through the working and round at the end.
Where this leads
Once masses are in moles, the balanced equation tells you the ratio in which substances react. That is covered on the page about mass calculations from equations. To convert your own figures, use the converter.
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.