Evaporation of a 20% aqueous solution yielded 36 g of dry matter. How many grams of water have evaporated?

Given:
m (substance) = 36 g
W = 20%
To find:
m (H2O) =?
Decision:
1) Let’s write the formulas by which we will solve.
W = m substance / m solution, hence m solution = m substance / W.
m solution = m substance + H2O
m (H2O) = m solution – m substance.
2) Find the mass of the entire solution.
m solution = 36: 0.2 = 180 g.
3) Find the mass of water.
m (H2O) = 180 g – 36 g = 144 g.
Answer: m (H2O) = 144 g.