How to Calculate Molar Mass From a Chemical Formula
To calculate molar mass, add up the standard atomic weights of every atom in the chemical formula. Multiply each element's atomic weight by how many of that atom appear, then total them. Water (H2O) is 2 × 1.008 + 15.999 = 18.015 g/mol, and glucose (C6H12O6) is 180.16 g/mol.
What Is Molar Mass?
Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol). It is numerically equal to the substance's molecular weight in atomic mass units. Molar mass is the bridge between mass you can weigh on a balance and the number of moles a reaction needs: grams divided by molar mass gives moles, and moles times molar mass gives grams.
Every molar-mass calculation is really just careful addition of atomic weights, which are listed on the periodic table.
Adding Up the Atomic Weights
Break the formula into its elements and count how many of each atom it contains. Multiply each count by that element's standard atomic weight — hydrogen is 1.008, carbon is 12.011, oxygen is 15.999, and so on — then add the products.
For glucose, C6H12O6: carbon is 6 × 12.011 = 72.066, hydrogen is 12 × 1.008 = 12.096, and oxygen is 6 × 15.999 = 95.994, totaling 180.156 g/mol. The atomic weights are averages over each element's natural mix of isotopes, so the result reflects a normal terrestrial sample.
Handling Parentheses and Subscripts
A subscript multiplies the atom it follows, and a group in parentheses is multiplied by the number after the closing parenthesis. In calcium hydroxide, Ca(OH)2, the OH group is doubled: one calcium (40.078) plus two oxygens (2 × 15.999) plus two hydrogens (2 × 1.008) equals 74.092 g/mol.
Work from the innermost group outward. For nested brackets, resolve the inner multiplier first, then apply the outer one — the same order you would use in ordinary arithmetic.