What is the difference between molar mass and molecular weight?

Relative molecular (formula) mass is the sum of the relative atomic masses in one formula unit and has no unit. Molar mass is the mass of one mole of the substance in g/mol. They have the same number: water has a relative molecular mass of 18.015 and a molar mass of 18.015 g/mol.

How do I calculate the molar mass of a hydrate?

Add the water of crystallisation. CuSO₄·5H₂O = 63.546 + 32.06 + 4 × 15.999 + 5 × (2 × 1.008 + 15.999) ≈ 249.68 g/mol. Type it with · or *, as in CuSO4·5H2O or CuSO4*5H2O; the number after the dot is the number of water molecules.

How is the mass percent of an element worked out?

Mass percent = number of atoms × atomic weight ÷ molar mass × 100%. In water, oxygen is 15.999 ÷ 18.015 ≈ 88.81%. The calculator lists this for every element, and the percentages add up to 100%.

Why is my answer slightly different from my textbook’s?

Textbooks often round atomic weights (H = 1, O = 16, Cl = 35.5). This calculator uses IUPAC standard atomic weights in abridged form (H 1.008, O 15.999, Cl 35.45), so the decimals can differ a little. Radioactive elements without a standard atomic weight use the mass number of their most stable isotope, shown in square brackets.

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🧫Molar Mass Calculator - Mass Percent, Grams to Moles

The molar mass M of a compound is the sum of the atomic weights of all atoms in its formula, in g/mol; numerically it equals the relative molecular (formula) mass. Water is 2 × 1.008 + 15.999 = 18.015 g/mol, glucose C₆H₁₂O₆ is 180.156 g/mol and Ca₃(PO₄)₂ is about 310.17 g/mol. Type a formula and the calculator adds up IUPAC standard atomic weights (abridged values) element by element, listing the atom count, mass contribution and mass percent of each element along with the full sum. Brackets can be nested, hydrates are written CuSO4·5H2O or CuSO4*5H2O, and ions such as SO4^2- are accepted. Below the result, a converter moves between mass m, amount n and number of particles N (n = m / M, N = n × Nₐ) — edit any box and the others update — and a solution helper tells you how much solute to weigh out for a target molarity and volume, or what concentration you get from a given mass.

Free · No sign-up · Runs in your browserLast updated: 2026-10-26

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How to use Molar Mass Calculator

1Enter a formula such as H2O, Ca3(PO4)2 or CuSO4·5H2O, or pick an example
2Read the molar mass, the elemental composition table and the worked sum
3Convert between grams, moles and particles, or enter a volume and concentration to plan a solution

FAQ

What is the difference between molar mass and molecular weight?: Relative molecular (formula) mass is the sum of the relative atomic masses in one formula unit and has no unit. Molar mass is the mass of one mole of the substance in g/mol. They have the same number: water has a relative molecular mass of 18.015 and a molar mass of 18.015 g/mol.
How do I calculate the molar mass of a hydrate?: Add the water of crystallisation. CuSO₄·5H₂O = 63.546 + 32.06 + 4 × 15.999 + 5 × (2 × 1.008 + 15.999) ≈ 249.68 g/mol. Type it with · or *, as in CuSO4·5H2O or CuSO4*5H2O; the number after the dot is the number of water molecules.
How is the mass percent of an element worked out?: Mass percent = number of atoms × atomic weight ÷ molar mass × 100%. In water, oxygen is 15.999 ÷ 18.015 ≈ 88.81%. The calculator lists this for every element, and the percentages add up to 100%.
Why is my answer slightly different from my textbook’s?: Textbooks often round atomic weights (H = 1, O = 16, Cl = 35.5). This calculator uses IUPAC standard atomic weights in abridged form (H 1.008, O 15.999, Cl 35.45), so the decimals can differ a little. Radioactive elements without a standard atomic weight use the mass number of their most stable isotope, shown in square brackets.