Solubility Calculations

Lesson Plan - Get It!

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Putting too much sugar in lemonade can make it too sweet, while not enough makes it tart and sour.

Knowing how much solute to put in a solution is an important skill. There are several ways to calculate the concentration of a substance. Remember that concentration means how much solute is dissolved into the solvent.

Before you continue, if you missed or would like to review the previous lessons in our Mixtures and Solutions series, find them in the right-hand sidebar under Related Lessons.

The first way to calculate the concentration is called "molarity." The abbreviation for molarity is an uppercase M. Molarity is calculated by taking the moles of solute and dividing it by the liters of total solution. Moles are calculated by comparing the amount of a substance to the molar mass of the substance. The molar mass of an element is found on the periodic table. For example, the mass of carbon is 12g for every one mole.

Take a look at an example problem for molarity.

• What is the molarity of 4.2 moles of sodium chloride dissolved into 2.5 liters of solution? Set up your equation: 4.2 moles/2.5 liters of solution. Divide the terms to calculate a molarity of 1.68M.

Solutions with higher molarity concentrations have more solute dissolved into the solvent. Lower molarities indicates less solute. However, a low molarity solution can still be dangerous. Strong acids can be corrosive and harmful, even with a molarity of 0.01M.

Another way to calculate the solution concentration is through the percent weight comparison. This type of calculation uses the mass of the solute divided by the mass of the solution, multiplied by one hundred. The units for mass must be the same for both the solute and solution.

An example of this type of problem would be:

• What is the percent by weight of a 350-gram solution containing 68 grams of calcium carbonate? Set up the equation using the formula: 100 x (mass of solute/mass of solution). The masses would be given in the problem, not calculated by the periodic table.

(68 grams of calcium carbonate/350 grams of solution) x 100= 19.4% calcium carbonate solution would form.

You can use the same formula to calculate the percent by volume as well. The equation is the same: 100 x (volume of solute/volume of solution). The units in this type equation would be related to volume, milliliters, liters, or another volume unit.

Each of these equations can provide you with valuable information about the solution.

When carrying out experiments in a chemistry lab, you will run across many concentrations given through the molarity. Molarity is one of the most common concentration units used in science. You may have seen percent concentrations on materials like rubbing alcohol at the drug store. Percent by volume and percent by weight provide a different sort of concentration value.

In the Got It? section, manipulate a solution in different ways and track how the concentration changes.