How to Calculate for Molarity With More Than One Volume Present

Learn how to calculate molarity when you have two or more volumes in one solution.

Molarity is calculated by dividing the number of moles of a solute by the number of liters of solution. The calculation is fairly straightforward with one volume present. Multiple volumes will change the calculation, but fortunately, under the normal assumptions of the molarity equation, the change is slight. Simply add the multiple volumes together to get the total volume of the solution in liters, and let this number serve as the denominator in the molarity equation.

Things You'll Need

• Calculator

Instructions

1. Divide the mass of the solute in grams by the molar mass of the solute (grams/mole) to calculate the number of moles of solute.

2. Divide the mass of the solute in grams by the density of the solute in grams per liter to calculate the volume of the solute.

3. Add the volumes of the solute and the multiple solvents together in liters.

4. Divide the number calculated in Step 1 by the number calculated in Step 3 to calculate molarity in moles per liter.

Tips & Warnings

• This tutorial will work in solutions with more than one solvent as well. Simply repeat the above steps for any of solutes in the solution.

• The basic assumption of the molarity equation is that the solute and solvent(s) do not interact in such a way as to reduce the actual volume below the theoretical volume (i.e. the sum of all volumes). Stated a different way, the molarity equation assumes that multiple volumes are additive. This simplifies calculations but it is not always true in practice.

References

• Southern Oregon University: Chemistry Problems