The formula
M = n / V
| Symbol | Quantity | Unit |
|---|---|---|
| M | Molarity | mol/l |
| n | Moles of solute | mol |
| V | Volume of solution | l |
What it means
Molarity counts particles rather than mass, which is what reactions actually care about. One mole is 6.022 × 10²³ particles, so a 1 M solution has that many dissolved units in every litre.
How to work it out by hand
- Work out the moles of solute
- Measure the total volume of solution in litres
- Divide the moles by the litres
What is worth knowing
The detail that catches people is 'per litre of solution', not per litre of solvent: you dissolve the solute first and then make up to the mark, because dissolving changes the volume. Molarity also varies with temperature, since the volume expands, which is why molality — moles per kilogram of solvent — is preferred for precise work. For everyday laboratory use the difference is small enough to ignore, and molarity's convenience wins.