By Contributor Updated Mar 24, 2022
In chemistry, a solute is often added to a solution. One of the most common tasks is to determine the concentration of that solute, expressed as the solution’s molarity.
Learn the core equation: M = moles of solute ÷ liters of solution. The symbol “M” represents the molarity, i.e., the number of moles of solute per liter of solution.
Convert the mass of the solute into moles. Use a periodic table to find the molar mass of the compound. For example, sodium chloride (NaCl) has a molar mass of 58.5 g mol⁻¹ (23 g for Na + 35.5 g for Cl). If you dissolve 24 g of NaCl, the number of moles is 24 g ÷ 58.5 g mol⁻¹ ≈ 0.41 mol.
Measure the total volume of the solution. The volume must be expressed in liters for the molarity calculation. In this example, 650 mL of solution equals 0.65 L.
Calculate the molarity: M = 0.41 mol ÷ 0.65 L ≈ 0.63 M (rounded to two decimal places).
Molarity is moles of solute per liter of solution. Convert grams to moles using the correct molar mass, then divide by the solution volume in liters.
Always convert milliliters to liters before performing the division.
