Answer

To get the molarity, you divide the moles of solute by the litres of solution.

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

For example, a 0.25 mol/L NaOH solution contains 0.25 mol of sodium hydroxide in every litre of solution.

To calculate the molarity of a solution, you need to know the number of moles of solute and the total volume of the solution.

To calculate molarity:

1. Calculate the number of moles of solute present.
2. Calculate the number of litres of solution present.
3. Divide the number of moles of solute by the number of litres of solution

EXAMPLE:

What is the molarity of a solution prepared by dissolving 15.0 g of NaOH in enough water to make a total of 225 mL of solution?

Solution:

1 mol of NaOH has a mass of 40.00 g, so

$\text{Moles of NaOH}=15.0\text{g NaOH}\times \frac{\text{1 mol NaOH}}{40.00\text{g NaOH}}=\text{0.375 mol NaOH}$

$\text{Litres of solution}=225\text{mL soln}\times \frac{\text{1 L soln}}{1000\text{mL soln}}=\text{0.225 L soln}$

$\text{Molarity}=\frac{\text{moles of solute}}{\text{litres of solution}}=\frac{\text{0.375 mol}}{\text{0.225 L}}=\text{1.67 mol/L}$