Molar mass is the mass of one mole of a substance, in grams per mole (g/mol). To find it, add up the standard atomic weight of every atom in the chemical formula. Water, H₂O, is 2 × 1.008 + 15.999 = 18.015 g/mol.
A mole is a counting word, like a dozen. One mole is 6.02214076 × 10²³ particles of something. Molar mass answers the practical question that follows: if I want that many particles, how much do I have to weigh out? The answer is on the periodic table, one element at a time, and the formula tells you how many of each to count.
The three steps
1. Count each element in the formula. Subscripts multiply the atom in front of them. A subscript after a bracket multiplies everything inside the bracket. So Ca(OH)₂ is one calcium, two oxygen and two hydrogen — the 2 applies to both atoms in the bracket, not just the hydrogen.
2. Multiply each count by that element's atomic weight. Take the number under the symbol on the periodic table. Carbon is 12.011, hydrogen 1.008, oxygen 15.999.
3. Add the products together. The total is the molar mass in grams per mole. It is worth keeping the decimals until the final step, then rounding once.
Worked example: water, H₂O
Two hydrogen atoms and one oxygen. Hydrogen: 2 × 1.008 = 2.016. Oxygen: 1 × 15.999 = 15.999. Add them: 2.016 + 15.999 = 18.015 g/mol. So a mole of water weighs about 18 grams, which is roughly a tablespoon.
Worked example: glucose, C₆H₁₂O₆
Six carbon, twelve hydrogen, six oxygen. Carbon: 6 × 12.011 = 72.066. Hydrogen: 12 × 1.008 = 12.096. Oxygen: 6 × 15.999 = 95.994. Adding those gives 180.156 g/mol. Notice that carbon and oxygen do nearly all the work: hydrogen is twelve of the twenty-four atoms but under 7% of the mass.
Worked example: calcium hydroxide, Ca(OH)₂
The bracket is the part people get wrong. Ca(OH)₂ means one calcium plus two OH groups, so two oxygen and two hydrogen. Calcium: 40.078. Oxygen: 2 × 15.999 = 31.998. Hydrogen: 2 × 1.008 = 2.016. Total: 74.092 g/mol.
Hydrates: the dot in CuSO₄·5H₂O
That dot is not a multiplication or a decimal point. It means the crystal carries water molecules along with it, and the number in front says how many. CuSO₄·5H₂O is one copper sulfate unit plus five whole water molecules: 159.61 + 5 × 18.015 = 249.68 g/mol. Leaving the water out is one of the commonest ways to get a lab calculation wrong, because it changes the answer by more than a third.
Molar mass, molecular weight, formula mass
These are the same number used in slightly different ways. Molecular weight (relative molecular mass) is a ratio against a carbon-12 standard, so it has no units. Molar mass is that same number in grams per mole. Formula mass is the term some teachers prefer for substances like NaCl that form a lattice rather than separate molecules. Water is 18.015 in all three.
Why the numbers on the periodic table have decimals
Because they are averages. Chlorine is a mix of chlorine-35 and chlorine-37 as it occurs in nature, so its standard atomic weight is 35.45 — between the two, and matching neither. That average is the right number for weighing out a bulk sample, which is exactly what molar mass is for. If you need a particular isotope instead, use the average atomic mass calculator to see how the weighting works.
Common mistakes
Watch the capital letters: Co is cobalt, but CO is carbon monoxide, and they differ by nearly 14 g/mol. Apply a bracket subscript to everything inside the bracket. Do not round each element's weight to a whole number before adding, or the error compounds. And remember that the subscript belongs to the atom immediately before it, so CH₃COOH is two carbons, four hydrogens and two oxygens.
What molar mass is for
Almost every quantitative chemistry calculation passes through it. To turn a weighed mass into moles you divide by the molar mass; to turn moles back into a mass you multiply. That is the whole of the grams to moles conversion, and it is why molar mass is usually the first thing you work out.
Check your answer
Type any formula below and compare it with your own working. The breakdown shows each element's subtotal, so you can see exactly where a disagreement comes from.