How to Calculate Molarities

Concentration quantifies the amount a substance dissolved in a solution. Molarity expresses the concentration as number of moles of the dissolved compound in one liter of the solution.
Molarity=number of moles/volume of solution (in liters). Concentration in these units is abbreviated as "M." As an example, calculate molarity of the solution made by dissolving 60 grams of sodium hydroxide (NaOH) in 400 ml of water (H2O).

Things You'll Need

• Calculator Periodic table of the chemical elements

Instructions

1. Calculate molecular masses (M) of compounds that are dissolved in the solution.
   The molecular mass is calculated as the sum of mass of all atoms in the molecule.
   Atomic weights of corresponding elements have to be taken from the periodic table of the chemical elements.
   In our example
   M(NaOH)=M(Na)+M(O)+M(H)=23+16+1=40 grams/mole.

2. Calculate amounts of the compounds in the solution in moles using the formula
   Number of moles=mass(compound)/molecular mass(compound).
   In our example
   Number of moles (NaOH)=(60 grams) /(40 g/mole)=1.5 moles.

3. Calculate molarities. For each dissolved compound the molarity is given as
   Molarity=Number of moles/Volume of solution (in liters)
   In our example, volume of the solution is 400ml that is 0.4 liters.
   Molarity (NaOH)=1.5 moles/0.4 liters= 3.75 moles/liter=3.75 M.

References

• "Chemistry", K.W. Whitten, R.E. Davis, L. Peck, and G.G. Stanley, Brooks Cole, Feb. 17, 2009.

Resources

• Periodic table of the chemical elements