Answer

Molarity is the concentration of a solution expressed as the number of moles of solute per litre of solution.

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

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

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:

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

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

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

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