Molarity Calculator

The formula this calculator uses

M = n / V

M

Molarity — moles of solute per litre of solution

n

Amount of solute in moles

V

Volume of the solution in litres, not the solvent

Mr

Molar mass in g/mol, used to turn a weighed mass into moles

How to check the result by hand

1. 1

   Decide whether you have moles or grams

   If the solute came off a balance you have grams and must convert; if you measured a standard solution you may already have moles. Everything downstream depends on getting the amount into moles first, because the formula is written in moles and will not accept grams.

2. 2

   Convert mass to moles when needed

   Divide grams by molar mass: 20 g at 40 g/mol is 0.5 mol. Molar mass is the sum of atomic masses, so check the formula of the compound before adding them together. A salt such as sodium chloride has two atoms to account for, not one.

3. 3

   Measure the volume of the finished solution in litres

   Fill to the mark after dissolving. A volume in millilitres must be divided by 1000, so 500 mL enters the formula as 0.5 L, not as 500. This is the single most frequent arithmetic slip, and it changes the answer by a factor of a thousand.

4. 4

   Divide moles by litres

   0.5 mol in 2 L gives 0.5 / 2 = 0.25 M. This single division is the whole calculation; everything else is unit hygiene and presentation. There is no constant to look up and no exponent to apply.

5. 5

   Restate the answer in a convenient scale and check it

   0.25 M is 500 mmol, or 0.00025 mol per millilitre. Multiplying molarity back by volume should return the moles you started with, a quick way to catch a slip. If the round trip does not return 0.5 mol, the volume or the moles were transcribed wrongly.