Find the mass fraction of sodium chloride in the solution obtained by dissolving 15 g of this salt in 235 ml of water.

Given: m (NaCl) = 15 g, V (H2O) = 235 ml.
Find: ω (NaCl) -?
Decision.
1. Let’s calculate the mass of water by the formula: m = ρ • V.
M (Н2О) = ρ (Н2О) • V (Н2О) = 1 g / ml • 235 ml = 235 g. We see that for water the volume is equal to the mass, therefore this action is not necessary.
2. Calculate the mass of the solution:
m (solution) = m (H2O) + m (NaCl) = 235 g + 15 g = 250 g.
3. Let’s calculate the mass fraction of sodium chloride in solution:
ω (NaCl) = m (NaCl) / m (solution) = 15 g: 250 g = 0.06, or,
multiplying this number by 100%, we get 6%.
Answer: 6%.