Concentration represents the amount of the compound dissolved in the solution. Molarity is the number of moles of a substance in 1 liter of the solution. Another unit of the concentration, weight percentage, refers to the ratio of the mass of the solute (a dissolved substance) to the mass of the solution. Converting between concentrations is frequently required for various problems in chemistry.
Determine atomic masses of elements that comprise the dissolved compound using the Periodic Table of the Elements. For example, if the compound in the solution is potassium chloride (KCl), the atomic mass of potassium (K) is 39 and that of chlorine (Cl) is 35.5.
Multiply the atomic mass by the number of the respective atoms in the molecule, and then sum up the products to calculate the molar mass In this example, the molar mass of KCl is 39 x 1 + 35.5 x 1 = 74.5.
Multiply the molar mass of the compound by the molarity to calculate the amount of the dissolved substance in one liter of the solution. For example, 0.5 M of KCl solution contains 74.5 x 0.5 = 37.25 g of the salt.
Multiply the density of the solution by 1,000 ml (1 liter) to calculate the mass of the 1L of the solution. For example, if the density of 0.5 M KCl solution is 1.1 g/ml, the weight of 1 liter of the solution is 1.1 x 1,000 = 1,100 g.
Divide the mass of the dissolved compound by the mass of the solution, and multiply the result by 100 to calculate percentage. In this example, the solution of KCl is (37.25 ÷ 1,100) x 100 = 3.39 percent.