How do I calculate molar mass?

Sum the atomic masses of every atom in the formula. Water is (2 × 1.008) + 15.999, giving 18.015 g/mol.

The amount of substance containing 6.022 × 10²³ particles. The definition makes molar mass in grams numerically equal to atomic mass in atomic mass units.

How do I convert grams to moles?

Divide the mass in grams by the molar mass. 36 g of water is 36 ÷ 18.015, which is about 2 moles.

What is the difference between empirical and molecular formula?

Empirical gives the simplest atom ratio, molecular the actual count. Glucose is CH₂O empirically but C₆H₁₂O₆ molecularly.

Molecular Weight Calculator - Molar Mass from Formula

Molecular Weight Calculator

Molar mass is the mass of one mole of a substance, found by adding up the atomic masses in its formula. It is the bridge between the grams you weigh and the moles chemistry works in.

Formula

Molar mass = Σ (atomic mass × number of those atoms)

A subscript multiplies only the atom before it
A subscript after a bracket multiplies everything inside it

H₂SO₄: (2 × 1.008) + 32.06 + (4 × 16.00) = 98.08 g/mol

How It Is Calculated

Add the atomic mass of every atom in the formula.

Water, H₂O: (2 × 1.008) + 15.999 = 18.015 g/mol.

Carbon dioxide, CO₂: 12.011 + (2 × 15.999) = 44.01 g/mol.

Subscripts multiply only the atom immediately before them, so brackets matter: Ca(OH)₂ contains two oxygens and two hydrogens, not one of each.

Grams, Moles and Avogadro

Moles = Mass (g) ÷ Molar mass (g/mol)

One mole contains 6.022 × 10²³ particles — Avogadro's number. The definition is chosen so that atomic mass in atomic mass units equals molar mass in grams per mole, which is what makes the arithmetic so convenient.

18 g of water is one mole, containing about 6 × 10²³ molecules.

Empirical and Molecular Formulas

An empirical formula gives the simplest whole-number ratio of atoms; a molecular formula gives the actual count.

Glucose is C₆H₁₂O₆ molecularly but CH₂O empirically. Both have the same percentage composition, so elemental analysis alone cannot distinguish them — you also need the molar mass, which is why mass spectrometry pairs with combustion analysis.

Quick Reference

Worked molar masses for common compounds.
Compound	Formula	Molar mass
Water	H₂O	18.02 g/mol
Table salt	NaCl	58.44 g/mol
Caustic soda	NaOH	40.00 g/mol
Sulfuric acid	H₂SO₄	98.08 g/mol
Glucose	C₆H₁₂O₆	180.16 g/mol
Slaked lime	Ca(OH)₂	74.09 g/mol

Frequently Asked Questions

How do I calculate molar mass?: Sum the atomic masses of every atom in the formula. Water is (2 × 1.008) + 15.999, giving 18.015 g/mol.
What is a mole?: The amount of substance containing 6.022 × 10²³ particles. The definition makes molar mass in grams numerically equal to atomic mass in atomic mass units.
How do I convert grams to moles?: Divide the mass in grams by the molar mass. 36 g of water is 36 ÷ 18.015, which is about 2 moles.
What is the difference between empirical and molecular formula?: Empirical gives the simplest atom ratio, molecular the actual count. Glucose is CH₂O empirically but C₆H₁₂O₆ molecularly.