How to calculate molarity

Molarity ($M$) measures the concentration of a solution: the amount of solute dissolved in a given volume of solution.

$M = \frac{\text{moles of solute}}{\text{litres of solution}}$

Worked example

What is the molarity of a solution made by dissolving 2.0 moles of sodium chloride in 4.0 litres of water?

$M = \frac{2.0\ \text{mol}}{4.0\ \text{L}} = 0.50\ \text{mol/L}$

So the solution is 0.50 M NaCl.

What if you are given grams instead of moles?

Convert grams to moles first using molar mass:

$\text{moles} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}}$

Then apply the molarity formula. Practise on randomised molarity problems below.