40 g of water was evaporated from 140 g of a 5% solution of baking soda. Find the mass fraction of soda in the resulting solution.

Given: m (solution) = 140 g, ω (soda) = 5%, or 0.05, m (water) = 40 g.
Find: ωп (soda) -?
Decision.
1. Find the mass of the dissolved substance (soda) in the solution:
m (soda) = m (solution) • ω (soda) = 140 g • 0.05 = 7 g.
2. Find the mass of the resulting solution:
mp (solution) = m (solution) – m (solution) = 140 g – 40 g = 100 g.
3. Find the mass fraction of the dissolved substance (soda) in the resulting solution:
ωp (soda) = m (soda) / mp (solution) = 7 g: 100 g = 0.07, or 7%.
Answer: ωp (soda) = 7%.

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