How to Calculate the Concentration of a Chemical Solution

Ways to calculate a concentration of a chemical solution depend on the purpose for which the the calculation is necessary. As an example, a chemist might be interested in knowing a specie concentration while a pharmacy technician may only be interest in percentage concentration. As a result there is varying terminology and methods of determining chemical solution concentration depending on our needs. For this discussion, let us focus on some common ways to calculate chemical solution concentration.

The first method is percentage concentration. Percentage concentration for solutions can be expressed in two ways, namely: volume-in-volume (v/v) and weight-in-volume(w/v)percentage preparations.Volume-in-volume (v/v) percentage applies when the final product is a liquid. We show that the final product is a liquid by the “v” in the denominator. The constituent for which the percentage is being measured is also a liquid. That is why we have another “v” in the numerator. Make sure the units in the numerator and denominator are the same, e.g. L/L.

Weight-in-volume (w/v) percentage applies when the final product is a liquid. We indicate that the final product is a liquid by the “v” in the denominator. The constituent for which the percentage is being measured is a solid. That is why we have a “w” in the numerator. Make sure the units in the numerator are always in grams and milliliters in the denominator, whereas in volume-in-volume the units are the same.

Another way to calculate concentration of a chemical solution is by determining its Molarity (M). Sometimes chemists would like to go further than just determining the analytical molarity of a solution and find out the equilibrium molarity of a solution. The major difference between the two calculations is that the analytical molarity of a solution tells the total number of moles of a solute in one liter of solution regardless of the chemical state of the solute, while the equilibrium molarity refers to the molar concentration of a particular specie in a solution at equilibrium. The two expressions still tell us how many moles (or millimoles) of a solute we have in a liter(milliter of solution) of solution.

Recap on how to calculate the number of moles: First recall that 1 mole of a substance is equivalent to its molar mass. Use ratios to determine how many moles there are in the given mass of the substance in question.

As with percentage calculations keep track of the units. Molarity is moles per liter and whatever you do make sure you end up with these units.