Mole Calculation Formula
The mole formula n = m/M converts between mass and moles using molar mass.
Fundamental for all chemistry stoichiometry calculations.
The Formula
n = m / M
The number of moles equals the mass of a substance divided by its molar mass. This formula is the bridge between the mass you can measure and the number of particles involved.
Variables
| Symbol | Meaning |
|---|---|
| n | Number of moles (mol) |
| m | Mass of the substance (measured in grams, g) |
| M | Molar mass (measured in grams per mole, g/mol) |
Example 1
How many moles are in 36 g of water (H₂O)? The molar mass of water is 18 g/mol.
Identify the values: m = 36 g, M = 18 g/mol
Apply the formula: n = m / M = 36 / 18
n = 2 mol
Example 2
You need 0.25 moles of sodium chloride (NaCl). The molar mass of NaCl is 58.44 g/mol. What mass do you need?
Rearrange: m = n × M
m = 0.25 × 58.44
m = 14.61 g
When to Use It
Use the mole calculation formula for nearly every quantitative chemistry problem.
- Converting between grams and moles for any substance
- Stoichiometry — determining reactant and product amounts
- Preparing solutions of known concentration
- Look up the molar mass on the periodic table (sum of atomic masses)
Key Notes
- Molar mass is found from the periodic table: sum the atomic masses of all atoms in the formula — H₂O = 2(1.008) + 15.999 = 18.015 g/mol; for ionic compounds like NaCl, add all ions: 22.990 + 35.453 = 58.443 g/mol
- Stoichiometry uses mole ratios from the balanced equation: in 2H₂ + O₂ → 2H₂O, 2 mol H₂ reacts with exactly 1 mol O₂ — the coefficients are the mole ratios, so n(H₂) = 2 × n(O₂) always
- All mole calculations form a chain: mass (g) → moles via n = m/M → particles via N = n × Nₐ — and in reverse; moles are the universal "currency" in chemistry that connects every measurable quantity to particle counts
- Molarity C = n/V (mol/L) builds on this formula — dilution calculations M₁V₁ = M₂V₂ work because the number of moles in solution is constant when you add pure solvent; this is the same n from n = m/M