The formula
A mole of any substance weighs its molar mass in grams. To go from a mass to an amount, divide by the molar mass; to go back, multiply:
moles = grams ÷ molar mass (g/mol)
grams = moles × molar mass
The molar mass is the sum of the atomic weights in the formula. Water, H2O, is 2 × 1.008 + 15.999 = 18.015 g/mol.
Grams to moles
How many moles are in 25 g of sodium chloride? NaCl is 22.990 + 35.45 = 58.44 g/mol, so 25 ÷ 58.44 = 0.428 mol.
Moles to grams
How much does 0.25 mol of glucose weigh? C6H12O6 is 180.156 g/mol, so 0.25 × 180.156 = 45.04 g.
Weighing out a solution
To make 500 mL of 0.1 M NaOH you need 0.1 mol/L × 0.5 L = 0.05 mol. At 39.997 g/mol that is 0.05 × 39.997 = 2.00 g. Dissolve it in less than 500 mL of water, then make up to the mark.
Number of molecules
Multiply moles by Avogadro's number, 6.022 × 1023 per mole. The 0.428 mol of NaCl above is 0.428 × 6.022 × 1023 = 2.58 × 1023 formula units.
Common mistakes
- Using the wrong form of a salt. Many reagents are sold as hydrates. Copper(II) sulfate is 159.6 g/mol, but the blue pentahydrate, CuSO4·5H2O, is 249.7 g/mol, so weighing the hydrate with the anhydrous molar mass gives about 36% too little copper sulfate. Use the formula weight on the bottle's label.
- Diatomic gases. Oxygen gas is O2, 31.998 g/mol, not 15.999.
- Milligrams and millimoles. mg ÷ g/mol gives mmol, so the units stay consistent if you change both together.
Calculators
The Grams to Moles Calculator converts either way for any formula and works out the molar mass for you. For the molar mass alone, with each element's share, use the Molar Mass Calculator; for the mass needed for a solution, the Molarity Calculator.