Molar Mass Calculator

What molar mass is

Molar mass is the weight of one mole of a substance — one Avogadro’s number of its molecules or formula units — expressed in grams per mole. It’s a fixed property that comes straight from the chemical formula: add up the atomic mass of every atom, and that sum is the molar mass. Water is two hydrogens (1.008 each) plus one oxygen (15.999), so 18.015 g/mol.

That single number is the exchange rate between the two ways chemistry measures matter: grams on a balance and moles in an equation. It also works as an identification tool. If you can weigh a sample and independently count its moles — by titrating it, by measuring how much it lowers a solvent’s freezing point, or by collecting it as a gas — then dividing mass by moles hands you the molar mass, and the molar mass narrows down what the substance can be.

The molar mass formula

is molar mass in g/mol, is the measured sample mass in grams, and is the amount in moles. Built from a formula instead, it’s the sum over every atom :

Worked example

The calculator’s defaults are a 180.16 g sample containing exactly 1 mole.

That value matches glucose, C₆H₁₂O₆. Check it from the formula:

The two routes agree, which is how you’d confirm an unknown really is glucose.

Molar mass of common compounds

Where molar mass is needed

• Every gram–mole conversion, which is the first step of most stoichiometry.
• Solution prep — turning a target molarity and volume into grams to weigh.
• Identifying unknowns in analytical chemistry from mass-and-moles data.
• Checking a synthesis produced the compound you intended, not a byproduct with a different formula weight.

Common mistakes

• Skipping parentheses multipliers. Al₂(SO₄)₃ has 3 sulfurs and 12 oxygens.
• Ignoring waters of hydration. The hydrate weighs more than the anhydrous salt.
• Using atomic mass for a diatomic element — N₂ is 28.014, not 14.007.
• Over-rounding atomic masses before summing.

To go from molar mass to an amount, use the mole calculator or grams to moles. For concentration, see the molarity calculator.

A formula with parentheses

Multiply everything inside the brackets by the subscript outside. Calcium hydroxide, Ca(OH)₂, has one calcium and two hydroxide groups:

A hydrate

The dot in a hydrate’s formula means “plus”. Copper(II) sulfate pentahydrate, CuSO₄·5H₂O:

Water makes up of the crystal’s mass. That’s why heating the blue crystals to drive off the water leaves a noticeably lighter white powder.

Molar mass of a gas from the ideal gas law

For a gas, you can find the moles from pressure, volume and temperature, then divide the mass by them. Rearranged, . A 0.50 g sample filling 0.280 L at 1.00 atm and 273.15 K gives:

That’s very close to argon’s 39.95 g/mol, so argon is the likely gas.