Question

What is the percent concentration of a 300-mm solution of CaCl2?

Answer 1

The solution is 3.22 % CaCl₂ by mass.

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")` × 100 %

Step 1 — Determine the mass of the water.

Assume that you have 1 kg of water.

Step 2 — Determine the mass of CaCl₂.

A 300 mm solution contains 300 mmol of solute dissolved in 1 kg of solvent.

Mass of CaCl₂ = 1 kg water ×

`(300"mmol CaCl₂")/(1"kg water") × (1"mol CaCl₂")/(1000"mmol CaCl₂") × (111.0"g CaCl₂")/(1"mol CaCl₂")` = 33.3 g CaCl₂

Step 3— Determine the total mass of the solution

mass of solution = mass of CaCl₂+ mass of H₂O = (33.3 + 1000) g = 1033.3 g

Step 4 — Determine percent composition by mass of the CaCl₂ solution.

% by mass = `("mass of CaCl₂")/("mass of solution")` × 100 % = `(33.3" g")/(1033.3" g")` × 100 % = 3.22 %

The percent composition by mass of the CaCl₂ solution is 3.22 %.