Molar mass of any formula, element by element

Type a chemical formula and get its molar mass, the share each element contributes, and a grams to moles converter. Brackets, hydrates and charged ions all work, and every number comes from IUPAC standard atomic weights. It is free, with no ads and no sign-up.

How to calculate molar mass, step by step

1. Type the formula in the Chemical formula box, or press Try one.
2. Read the table: each element, its number of atoms, atomic weight, mass and share.
3. To convert, type grams to get moles, or moles to get grams.
4. Press Copy the result to paste it into your notes.

Writing the formula

Capital letters start each element, so Co is cobalt and CO is carbon monoxide. Write hydrates with * or a dot, like CuSO4*5H2O, and charges at the end, like SO4^2- or NH4+. Use brackets for groups, such as Ca(OH)2 or Al2(SO4)3.

How molar mass is worked out

Count the atoms of each element in the formula, multiply each count by that element’s standard atomic weight, and add them up. For water, H₂O, that is 2 × 1.008 + 15.999 = 18.015 g/mol. A number after a bracket multiplies everything inside it, so Ca(OH)₂ has two oxygens and two hydrogens.

Hydrates and ions

A hydrate like copper sulfate pentahydrate, CuSO₄·5H₂O, includes the water locked in its crystals: the 5 multiplies the whole H₂O. For an ion, the charge changes the mass by the mass of the electrons gained or lost, which is tiny but included. Elements with no stable isotopes use the mass number of their longest-lived isotope, so the result is marked as approximate.

Percent composition and moles

The share column is the mass percent of each element, the figure used to find empirical formulas from lab results. Type grams to get moles, or moles to get grams; moles are grams divided by the molar mass. Everything is worked out in your browser.

Tips for chemistry homework

• Check your capitals first. A wrong letter case is the most common reason for a surprising answer.
• Include the water in a hydrate if your question says to.
• Keep a few extra digits while you work and round only at the end.
• Use the percent composition to check a lab result against a known compound.

For other chemistry reference, see the periodic table.

Questions

What is the unit of molar mass?

Grams per mole, written g/mol.

How do I turn grams into moles?

Divide the mass in grams by the molar mass. The converter does it for you.

Why does my answer differ slightly from my textbook?

Textbooks round atomic weights differently. This tool uses IUPAC standard atomic weights, so small differences in the last digits are normal.