Weight Percent
Another similar way of expressing the concentration of a solution is to express it in weight percent (or mass percent, if you prefer).
Weight percent =
weight of solute
weight of solution
x 100%
You've done this kind of calculation earlier - last term in the lesson on composition. You may already be prepared to answer the questions in exercise 2 in your workbook. If so, do that and check your answers at the bottom of the page. If not, read on.
As an example, let's consider a 12% by weight sodium chloride solution. Such a solution would have 12 grams of sodium chloride for every 100 grams of solution. To make such a solution, you could weigh out 12 grams of sodium chloride, and then add 88 grams of water, so that the total mass for the solution is 100 grams. Since mass (unlike volume) is conserved, the masses of the components of the solution, the solute and the solvent, will add up to the total mass of the solution. 12 % NaCl solution = 12 g NaCl
100 g solution
12 g NaCl
(12 g NaCl + 88 g water) = 12% NaCl solution
To calculate the mass percent or weight percent of a solution, you must divide the mass of the solute by the mass of the solution (both the solute and the solvent together) and then multiply by 100 to change it into percent.