Molar Mass
Type a formula such as Ca3(PO4)2 or CuSO4·5H2O
| Element | Atoms | Atomic mass | Subtotal | % by mass |
|---|
Atomic masses follow the periodic table in the GenChemCalc reference tables. Methodology & sources →
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The molar mass of a compound is the sum of the atomic masses of all the atoms in its formula. For every element, multiply its atomic mass by the number of atoms present, then add the subtotals together.
Worked example — calcium phosphate, Ca3(PO4)2:
- Ca: 3 × 40.08 = 120.24
- P: 2 × 30.97 = 61.94
- O: 8 × 16.00 = 128.00
- Total = 310.18 g/mol
Once you have the molar mass you can convert between mass and moles with n = m ÷ M, which is exactly what the mini converter above does. Need the full method? See How to Approach Stoichiometry.
Worked Example — Molar Mass, Moles, and Particle Counts
Question: Find the molar mass of calcium phosphate, Ca₃(PO₄)₂. Then find (a) how many moles are in 85.0 g, and (b) how many grams correspond to 3.40 × 10²³ formula units.
Step 1 — molar mass: M = 3(40.08) + 2(30.97) + 8(16.00) = 120.24 + 61.94 + 128.00 = 310.18 g/mol
Step 2 (a) — moles in 85.0 g: n = m ÷ M = 85.0 ÷ 310.18 = 0.274 mol
Step 3 (b) — moles from particle count: n = 3.40 × 10²³ ÷ 6.022 × 10²³ = 0.565 mol
Step 4 (b) — convert to mass: m = 0.565 × 310.18 = 175 g
Answer: M = 310.18 g/mol; (a) 0.274 mol; (b) 175 g.
Common Mistakes
- Subscripts outside parentheses multiply everything inside. In Ca₃(PO₄)₂, the 2 applies to both P and all 4 O atoms — giving 2 P and 8 O, not 2 P and 4 O.
- Rounding atomic masses too early. Use at least 2 decimal places from the periodic table throughout the calculation, and round only the final answer — early rounding compounds across multi-step problems.
- Hydrates need the water included. CuSO₄·5H₂O has a molar mass of 249.69 g/mol, which includes the five water molecules — not just the 159.61 g/mol of anhydrous CuSO₄.
Frequently Asked Questions
Add up the atomic mass of every atom in the formula. For each element, multiply its atomic mass by the number of atoms of it, then sum the results. For example, H₂O = 2 × 1.008 + 1 × 16.00 = 18.02 g/mol.
They are numerically identical. Molecular weight (formula mass) is the mass of one molecule in atomic mass units; molar mass is the mass of one mole in grams per mole. Same number, so this tool answers both.
Put a dot before the water part: CuSO4.5H2O or CuSO4·5H2O. The number after the dot multiplies the whole water group, giving 249.69 g/mol for copper(II) sulfate pentahydrate.
For practical purposes they're the same number with different units: molecular weight is a dimensionless ratio (relative to 1/12 the mass of carbon-12), while molar mass is expressed in grams per mole. For ionic compounds, 'formula mass' is sometimes used instead of 'molecular mass' since ionic compounds don't form discrete molecules — but the calculation is identical.
Enter the formula with the water included, using a middle dot before the water (CuSO4.5H2O works in this calculator). The calculator adds the molar mass of 5 H2O (5 × 18.02 = 90.08 g/mol) to the anhydrous compound's mass.