Molar Mass Calculator

Molar Mass Explained

Molar mass is the mass of exactly one mole (6.022 × 10²³ particles) of a substance, found by adding up the atomic masses of every atom in its chemical formula.

$$M = \sum (\text{atomic mass} \times \text{count})$$

Each element's contribution is its atomic mass (from the periodic table) multiplied by how many times it appears in the formula — parentheses group atoms that repeat together as a unit, like the two OH groups in Ca(OH)₂.

Worked Example: Calcium Hydroxide

Using the calculator's default formula, Ca(OH)₂ (calcium hydroxide, also known as slaked lime) — one calcium atom (40.08 g/mol), two oxygen atoms (2 × 16.00 = 32.00 g/mol), and two hydrogen atoms (2 × 1.008 = 2.016 g/mol) — adds up to a molar mass of about 74.10 g/mol. This value is exactly what you'd need to convert between a measured mass of the compound and the number of moles present for a reaction calculation.

Why Molar Mass Is the Bridge Between Mass and Moles

Chemical reactions occur based on the number of molecules reacting, not their mass — but mass is what's actually measured on a lab balance. Molar mass is the conversion factor connecting the two: divide a measured mass by molar mass to get moles, or multiply moles by molar mass to get mass. This single calculation underlies essentially all quantitative chemistry, from simple concentration calculations to full reaction stoichiometry.

A Brief History of Atomic Mass

John Dalton proposed the first table of relative atomic weights in 1803, though his values were often inaccurate by modern standards since he lacked precise ways to determine atomic ratios in compounds. The scale was refined repeatedly through the 19th and 20th centuries, eventually standardized against carbon-12 in 1961 by international agreement, giving the modern atomic mass values used in every periodic table today.

Common Molar Mass Mistakes

Forgetting that a subscript outside a closing parenthesis multiplies everything inside is a common error — in Ca(OH)₂, that trailing 2 applies to both the O and the H inside the parentheses, not just to the H immediately before it. Misreading element symbols (confusing Co, cobalt, with CO, carbon monoxide, since case matters enormously in chemical notation) is another frequent mistake. Using average atomic mass when a specific isotope's exact mass was intended is a third subtlety — standard molar mass calculations always use the naturally-occurring average, which is correct for virtually all everyday chemistry.

Molar Mass Terms You Should Know

Mole — a unit representing exactly 6.022 × 10²³ particles (Avogadro's number).

Atomic Mass — the average mass of one atom of an element, in atomic mass units (u) or g/mol.

Molecular Formula — a formula showing the actual number of each atom in one molecule of a compound.

Formula Unit — the smallest repeating ratio of ions in an ionic compound, analogous to a molecule for covalent compounds.

This calculator does not parse hydrate notation (the middle dot in compounds like CuSO₄·5H₂O) — see the FAQ below for how to handle those manually.