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)