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:
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##