What this calculator does
Molarity is moles of solute per litre of solution. Ten grams of sodium chloride, molar mass 58.44, made up to 500 mL gives 0.3422 mol/L.
The phrase that matters is "made up to". Molarity is defined by the final volume of the solution, not the volume of solvent added. Dissolving solute changes the volume, sometimes substantially, so the correct procedure is to dissolve in less than the target volume and then top up to the mark, rather than adding solute to a measured volume of water.
The formula
The mass is divided by the molar mass to give moles, which is divided by the volume in litres. The mass concentration in grams per litre is reported alongside, which is often the more practical figure when preparing a solution by weighing.
| Term | Meaning |
|---|---|
| Molarity (M) | Moles of solute per litre of solution. |
| Molar mass | Grams per mole, the sum of atomic masses in the formula. |
| Volumetric flask | Glassware calibrated to a single volume, used to make solutions up to the mark. |
| Molality | The related measure using kilograms of solvent instead, which does not change with temperature. |
The inputs explained
| Field | What to enter |
|---|---|
| Mass of solute (g) | Mass of solute weighed out, in grams. |
| Molar mass of solute (g/mol) | Molar mass of the solute. Sodium chloride is 58.44 g/mol; water is 18.02. |
| Final volume of solution (mL) | Final volume of the finished solution in millilitres, not the volume of solvent added. |
When to use it
Preparing a solution
Working out what mass to weigh for a target concentration is the everyday use.
Checking a recipe
Confirming that a published preparation gives the concentration it claims.
Converting between concentration units
Molarity, grams per litre and ppm all describe the same solution differently.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does volume change the concentration?
The same mass of solute made up to different volumes.
| Final volume | Molarity | Moles of solute | Mass concentration |
|---|---|---|---|
| 250 mL | 0.6845 mol/L | 0.1711 mol | 40.000 g/L |
| 500 mL | 0.3422 mol/L | 0.1711 mol | 20.000 g/L |
| 1000 mL | 0.1711 mol/L | 0.1711 mol | 10.000 g/L |
| 2000 mL | 0.0856 mol/L | 0.1711 mol | 5.000 g/L |
Questions
What is the difference between molarity and molality?
Molarity is per litre of solution, molality is per kilogram of solvent. Molarity changes with temperature because volume does; molality does not, since mass is temperature-independent. Molality is therefore used for colligative property calculations.
Do I add solute to a litre of water?
No. Dissolve the solute in less than the target volume, then make up to the mark. Adding solute to a full litre gives more than a litre of solution and therefore a concentration below what you intended. For dilute solutions the error is small; for concentrated ones it is not.
How do I find the molar mass?
Add the atomic masses of every atom in the formula. Sodium chloride is 22.99 plus 35.45, giving 58.44. For anything complicated, use a molecular weight calculator rather than doing it by hand, since a single arithmetic slip carries through everything downstream.
What does 1 M mean?
One mole of solute per litre of solution. For sodium chloride that is 58.44 g made up to a litre. The capital M is the standard symbol, and it is read as molar, so 0.5 M is described as half molar.
For diluting a stock solution, see the dilution calculator. For the temperature-independent measure, see the molality calculator.