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# What is 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" = "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:

- Calculate the number of moles of solute present.
- Calculate the number of litres of solution present.
- 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

##"Moles of NaOH" = 15.0 cancel("g NaOH") × "1 mol NaOH"/(40.00 cancel("g NaOH")) = "0.375 mol NaOH"##

##"Litres of solution" = 225 cancel("mL soln") × "1 L soln"/(1000 cancel("mL soln")) = "0.225 L soln"##

##"Molarity" = "moles of solute"/"litres of solution" = "0.375 mol"/"0.225 L" = "1.67 mol/L"##

Some students prefer to use a "molarity triangle".

It summarizes the molarity formulas as

##"Moles" = "molarity × litres"##

##"Molarity" = "moles"/"litres"##

##"Litres" = "moles"/"molarity"##