How to calculate molar mass
The molar mass (M) of a compound is the mass of one mole of it, in grams per mole (g/mol). It is numerically the same as the molecular weight in atomic mass units, so the two terms are often used interchangeably.
- Count the atoms of each element in the formula. A subscript after a bracket multiplies everything inside it: in Ca(OH)2 there is 1 Ca, 2 O and 2 H.
- Multiply each count by the element's atomic mass from the periodic table.
- Add all the contributions.
Example · glucose, C₆H₁₂O₆
C: 6 × 12.011 = 72.066 · H: 12 × 1.008 = 12.096 · O: 6 × 15.999 = 95.994. Molar mass = 180.156 g/mol. Carbon makes up 72.066 / 180.156 = 40.0% of its mass.
From grams to moles
Once you know M, converting between mass and amount of substance is one division: n = m / M. 9 g of water (M = 18.015 g/mol) is 0.4996 mol, which is about 3.01 × 10²³ molecules (multiply by Avogadro's number, 6.022 × 10²³). Enter a sample mass and the calculator does this for you.
Writing formulas
- Element symbols start with a capital letter:
Cois cobalt,COis carbon monoxide. - Use round or square brackets for groups:
Al2(SO4)3,K4[Fe(CN)6]. - For hydrates use a dot or an asterisk:
CuSO4·5H2OorCuSO4*5H2O.
Frequently asked questions
Which atomic masses does it use?
The standard atomic weights recommended by IUPAC, rounded to three decimals. Your textbook may round differently (for example O = 16), so small differences in the last decimal are normal.
How do I make a solution of a given molarity?
Use the molarity calculator: type the formula and it works out the molar mass and the grams you need.