ChemBench

Molar Mass Calculator

Type a formula, get the molar mass.

Chemical formula (e.g. H2O, C6H12O6, Ca(OH)2)

Molar mass

18.015 g/mol

H: 2 × 1.008 = 2.016
O: 1 × 15.999 = 15.999

What you entered

Formula: H2O

1
H × 2
1.008 × 2= 2.016 g/mol
2
O × 1
15.999 × 1= 15.999 g/mol
3
Total molar mass
2.016 + 15.999= 18.015 g/mol

Result

Molar mass: 18.015 g/mol

H2O has a molar mass of 18.015 g/mol — the mass of one mole (6.022×10²³ molecules) of this compound.

Molar mass of common compounds

Molar mass is the sum of every atom's standard atomic weight, in grams per mole. It is the number that converts between grams in a flask and moles in an equation.

Compound	Formula	Molar mass	Made up of
Hydrogen gas	H2	2.016 g/mol	H x2 = 2.02
Methane	CH4	16.043 g/mol	C 12.01 + H x4 4.03
Ammonia	NH3	17.031 g/mol	N 14.01 + H x3 3.02
Water	H2O	18.015 g/mol	H x2 2.02 + O 16.00
Oxygen gas	O2	31.998 g/mol	O x2 = 32.00
Carbon dioxide	CO2	44.009 g/mol	C 12.01 + O x2 32.00
Ethanol	C2H5OH	46.069 g/mol	C x2 24.02 + H x6 6.05 + O 16.00
Sodium chloride	NaCl	58.440 g/mol	Na 22.99 + Cl 35.45
Sulfuric acid	H2SO4	98.072 g/mol	H x2 2.02 + S 32.06 + O x4 64.00
Calcium carbonate	CaCO3	100.086 g/mol	Ca 40.08 + C 12.01 + O x3 48.00
Glucose	C6H12O6	180.156 g/mol	C x6 72.07 + H x12 12.10 + O x6 95.99

Atomic weights are IUPAC standard values averaged over natural isotope abundance, which is why almost none of them are whole numbers.

Adding up atomic weights

A compound's molar mass is the sum of the atomic weights of every atom in its formula — water (H2O) is 2 hydrogens (1.008 each) plus 1 oxygen (15.999), totaling about 18.015 g/mol.

Why chemists need this constantly

Molar mass is the bridge between the lab scale (grams, which you can weigh) and the molecular scale (moles, which is how chemistry actually happens) — it's used in nearly every stoichiometry calculation.

Frequently asked questions

I need to weigh out 0.25 mol of glucose (C6H12O6) — how many grams is that?

Glucose has a molar mass of about 180.16 g/mol, so 0.25 mol is 0.25 × 180.16 = 45.04 g. Weigh out 45.04 g on your balance.

How is molar mass different from molecular weight?

They give the same number but molar mass has units of g/mol (mass per mole of substance) while molecular weight is a dimensionless ratio relative to 1/12 of carbon-12. In practice, chemists use them interchangeably.

Does molar mass change for isotopes?

Yes. The periodic table lists average atomic masses weighted by natural isotope abundance, but if you use a pure isotope (like deuterium instead of hydrogen), the molar mass shifts. Heavy water (D2O) has a molar mass of about 20.03 g/mol versus 18.015 for regular water.

How do I handle parentheses in a formula like Ca(OH)2?

Multiply everything inside the parentheses by the subscript outside. Ca(OH)2 = 1 calcium (40.08) + 2 oxygens (2 × 16.00) + 2 hydrogens (2 × 1.008) = 74.10 g/mol.