You divide by the mass of the solution.
Percent by mass is the mass of the solute divided by the mass of the solution, multiplied by 100.
Example:
You dissolve a 4.00 g sugar cube in 350 mL of tea at 80 °C. The density of water at 80 °C is 0.975 g/mL. What is the percent by mass of sugar in the solution?
Solution
Step 1 — Determine mass of the solute.
The solute is the sugar cube. Mass of solute = 4.00 g.
Step 2 — Determine mass of solvent.
The solvent is the 80 °C water. Use the density of the water to find the mass.
mass of solvent = 350 mL ×
0.975
g
1
mL
= 341 g
Step 3— Determine the total mass of the solution
mass of solution = mass of solute + mass of solvent = 4.00 g + 341 g = 345 g
Step 4 — Determine percent composition by mass of the sugar solution.
% by mass =
mass of solute
mass of solution
× 100 % =
4.00
g
345
g
× 100 % = 1.159 %
The percent composition by mass of the sugar solution is 1.159 %.
