How to calculate molar mass
- Type a chemical formula, such as H2O, C6H12O6 or Ca(OH)2. Capitals matter: Co is cobalt, CO is carbon monoxide.
- Brackets, square brackets and hydrates are supported: write copper sulfate pentahydrate as CuSO4·5H2O or CuSO4.5H2O.
- The molar mass appears instantly with a breakdown by element and the mass percentage of each. Open "Grams ⇄ moles" to convert a mass to moles and back.
What molar mass means
The molar mass of a substance is the mass of one mole of it, in grams per mole. A mole is 6.022 × 10²³ particles (Avogadro's number), chosen so that the molar mass in grams equals the relative formula mass. To calculate it, multiply each element's atomic mass by the number of atoms of that element in the formula and add them up. For water, H2O: 2 × 1.008 + 15.999 = 18.015 g/mol.
Molar mass is the bridge between the mass you weigh on a balance and the number of particles that react. Moles = mass ÷ molar mass, so 10 g of glucose (180.156 g/mol) is 0.0555 mol. The mass percentages give the percentage composition, used in empirical formula problems and to check purity. This calculator uses IUPAC standard atomic weights rounded to five significant figures, which is more precise than most school data sheets; your exam board's table may give slightly different answers in the last decimal place. Everything runs in your browser.
What is the molar mass of water?
18.015 g/mol: two hydrogen atoms (2 × 1.008) plus one oxygen atom (15.999).
How do I enter a hydrate?
Use a dot between the parts, for example CuSO4·5H2O or CuSO4.5H2O. The number after the dot multiplies the water.
Is molar mass the same as molecular weight?
Numerically yes. Molecular weight (relative molecular mass) has no units; molar mass is in grams per mole.