Molarity is an unit for expressing the concentration of a solute in a solution, and it is calculated by dividing the moles of solute by the liters of solution. Written in equation form, molarity = moles of solute / liters of solution.
- Calculate the moles of solute present
Calculate the number of moles from the given data by dividing the amount of solute by the molecular weight of the solute and multiplying it by 1 mol. For example, sodium hydroxide (NaOH) has a molecular weight of 40 g. So if a solution contains 20 g of NaOH, the moles of solute present is 0.5, and it's written as 0.5 mol NaOH.
- Calculate the number of liters of solution
If the volume of the solution is given in any unit other than liters, then convert that unit to liters of solution. For example, if a solution has a volume of 500 ml, converting it to liters of solution yields the result of 0.5 liters of solution, or 0.5 L solution.
- Calculate the molarity of the solution
After calculating the number of moles of solute and the liters of solution, divide the former by the latter to calculate the molarity of the solution. For example, if a 500 ml solution of NaOH contains 20 g of NaOH, the molarity of the solution is 1 M, or 1 mol/L.