How to Calculate the Molar Concentration of a Solution

A concentration represents the amount of a dissolved compound (solute) in a solution. The molar concentration or molarity (M) reports the number of moles of the solute in one liter (L) of the solution. As an example, the molarity of 160 g of sodium perchlorate (NaClO4) dissolved in 400 ml of distilled water will be calculated.

Things You'll Need

• Periodic table of the elements
• Calculator

Instructions

1. Find the atomic weights of the elements that compose the dissolved compound from the periodic table of the elements (see Resources). In the example, the atomic weights of sodium (Na), chlorine (Cl) and oxygen (O) are 23, 35.5 and 16 respectively.

2. Add the atomic weights of all atoms in the molecule to compute its molecular mass.
   In this example, the molecular mass of NaClO4 is 23 + 35.5 + (4 x 16) = 122.5 g/mole.

3. Divide the mass of the dissolved compound by its molecular mass to calculate the number of moles. In the example, the number of moles of NaCO4 is 160 g / 122.5 g/mole = 1.3 moles.

4. Divide the volume of the solution (in ml) by 1,000 to convert it to liters (L). In the example, 400 ml will convert to 0.4 L.

5. Divide the number of moles of the dissolved compound by the solution volume to calculate the molar concentration. In the example, the molar concentration is 1.3 moles / 0.4 L = 3.3 M.

References

• “Chemistry”; K.W. Whitten, R.E. Davis, L. Peck and G.G. Stanley; 2009

Resources

• Las Alamos National Laboratory: Periodic Table of the Elements