From 100 grams of 5% salt solution, 20 grams of water was evaporated. What is the mass fraction of salt in the solution?

Given:
m1 solution (salt) = 100 g
ω1 (salt) = 5%
Δm (water) = 20 g

To find:
ω2 (salt) -?

Decision:
1) Calculate the mass of salt (will not change):
m (salt) = ω1 (salt) * m1 solution (salt) / 100% = 5% * 100/100% = 5 g;
2) Calculate the mass of the new salt solution:
m2 solution (salt) = m1 solution (salt) – Δm (water) = 100 – 20 = 80 g;
3) Calculate the mass fraction of salt in the resulting solution:
ω2 (salt) = m (salt) * 100% / m2 solution (salt) = 5 * 100% / 80 = 6.25%.

Answer: The mass fraction of salt in the resulting solution is 6.25%.