Molar Mass Calculations
Simple Explanation
Molar mass (M) is the mass of one mole of a substance, in grams per mole (g/mol) — numerically equal to its relative formula mass. The number of moles (n) in a sample is found by dividing its mass (m) by its molar mass.
Why Do We Need It?
This formula is the bridge between the mass you can actually measure on a balance and the number of moles (particles) chemists need for calculations.
Formula
Moles Formula
n = m/M
The number of moles in a sample, from its mass and molar mass.
- n
- — amount of substance, in moles (mol)
- m
- — mass of the sample, in grams (g)
- M
- — molar mass, in grams per mole (g/mol) — numerically equal to Mr
When to use it: Whenever you need to convert between a measured mass and the number of moles it represents, or vice versa.
Worked Example
Find the number of moles from a mass
How many moles are in 36 g of water? (Molar mass of water = 18 g/mol)
Why Does This Work?
Since molar mass tells you exactly how many grams make up one mole, dividing a sample's total mass by that figure tells you how many of those 'one-mole portions' are in the sample.
Real-Life Example
Measuring out a reaction in the lab
A chemist needs exactly 0.5 mol of a compound for an experiment and only has a balance, not a mole-counter.
Rearranging n = m/M to m = n × M tells them exactly how many grams to weigh out.
Practice
How many moles are in 88 g of carbon dioxide, CO₂? (Molar mass = 44 g/mol)
EasyWhat is the mass of 0.25 mol of water? (Molar mass = 18 g/mol)
MediumCommon mistake
Mixing up mass and molar mass — molar mass is a fixed property of the substance (g per mole), while mass is how much of the sample you actually have (in grams).
Quick Review
- n = m/M
- M (molar mass) is numerically equal to Mr, in g/mol.
- Rearrange to m = n × M to find mass instead.