How to Calculate the Molarity of the Solute

The solute is a compound (e.g. sugar) that is dissolved in the solvent (e.g. water). Concentration typically reports on an amount of the solute in the solution and is calculated using several different methods. Molarity represents the concentration as number of moles of the solute in one liter of the solution and is abbreviated as "M." Molarity=number of moles/Volume of solution (in liters).
As an example, the following example calculates molarity of sodium nitrate (NaNO3) in the solution made by dissolving 100 grams of the salt in 350 ml of water (H2O).

Things You'll Need

• Periodic table of the chemical elements Calculator

Instructions

1. Calculate the molecular mass of the solute as the sum of the mass of all atoms in the molecule. Atomic weights of corresponding elements are given in the periodic table of the chemical elements (see resources).
   In our example it would
   M (NaNO3) = M(Na)+ M(N)+ 3xM(O) = 23+14+(3x16) = 85 g/mole

2. Calculate the number of moles of the solute in the solution using the formula:
   Number of moles = mass (solute)/molecular mass (solute).
   In our example :
   Number of moles (NaNO3) = (100g)/(85g/mole) = 1.176 moles

3. Calculate molarity of the solute.
   Molarity = Number of moles/Volume of solution (in liters)
   In our example, volume of the solution is 350ml that is 0.35 L.
   Molarity (NaNO3) = 1.176 moles/0.35 L = 3.36 moles/L = 3.36 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