What do you divide the mass of to calculate the percent concentration of a solution?

1 Answer
May 1, 2014

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.

% by mass = #("mass of solute")/("mass of solution")# × 100 % = #("mass of solute")/("mass of solute + mass of [solvent](http://socratic.org/chemistry/solutions-and-their-behavior/solvent)")# × 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 %.