Molar Mass Calculator

What Is Molar Mass?

Molar mass (also called molecular weight) is the mass of one mole of a substance, expressed in grams per mole (g/mol). One mole contains exactly 6.022 × 10²³ particles (Avogadro's number). For a compound, the molar mass is the sum of the atomic masses of all atoms in the chemical formula, each multiplied by how many times that atom appears.

For example, water (H₂O) has 2 hydrogen atoms (1.008 g/mol each) and 1 oxygen atom (15.999 g/mol), giving a molar mass of 2(1.008) + 15.999 = 18.015 g/mol. This number is crucial for converting between grams and moles in stoichiometry problems.

How to Enter a Chemical Formula

• Element symbols — Use the standard one- or two-letter symbol with the first letter capitalized: H, Na, Fe, Cl.
• Subscripts — Type numbers directly after the element symbol: H2O means two hydrogen atoms and one oxygen atom.
• Parentheses and brackets — Group atoms that repeat: Ca(OH)2 means one calcium, two oxygen, and two hydrogen. Both () and [] are supported.
• No spaces or charges — Enter only the neutral empirical or molecular formula. Ionic charge notation (Fe3+) and isotope notation are not supported.

Common Uses for Molar Mass

• Stoichiometry — Converting grams of a reactant or product to moles using the formula: moles = mass ÷ molar mass.
• Solution preparation — Calculating how many grams of a solute to weigh out to prepare a solution of a specific molarity.
• Empirical and molecular formulas — Determining the molecular formula from percent composition data requires knowing the approximate molar mass from experiment.
• Ideal gas law — The ideal gas law calculator uses moles (n); you calculate moles from mass and molar mass before using it.

Frequently Asked Questions

What is the difference between molar mass and molecular mass?

Molecular mass (or molecular weight) is technically the mass of one molecule in atomic mass units (amu or u). Molar mass is the mass of one mole of molecules in grams per mole (g/mol). Numerically the values are identical — the molar mass of water is 18.015 g/mol and its molecular mass is 18.015 u. In practice the two terms are often used interchangeably in introductory chemistry.

Why are atomic masses not whole numbers?

Elements in nature exist as a mixture of isotopes — atoms of the same element with different numbers of neutrons. The atomic mass listed on the periodic table is a weighted average of the masses of all naturally occurring isotopes, weighted by their natural abundance. For example, chlorine has two main isotopes: ³⁵Cl (~75%) and ³⁷Cl (~25%), giving an average atomic mass of 35.453.

How many significant figures should I use?

In most general chemistry problems, molar masses are reported to 2–4 decimal places. This calculator uses IUPAC 2021 standard atomic weights. For exam work, rounding to one decimal place (e.g., H = 1.0, O = 16.0, C = 12.0) is usually acceptable unless the problem specifies otherwise. You can count and round your answers with the significant figures calculator.

Can I enter hydrate formulas like CuSO₄·5H₂O?

Hydrate dot notation is not currently supported. To work around this, expand the formula manually: CuSO₄·5H₂O is equivalent to CuSO4(H2O)5, which this calculator handles correctly.

Does this work for ionic compounds and polyatomic ions?

Yes — as long as you enter the neutral formula unit, not the ionic form. Enter NaCl, CaCO3, or Al2(SO4)3. Ionic charge notation such as SO4(2-) is not accepted; just enter the atoms as they appear in the formula unit.