30 g of potassium chloride was dissolved in 400 g of water. Determine the mass fraction of potassium chloride in the solution.

Given:
m (KCl) = 30 g
m (H2O) = 400 g

To find:
ω (KCl) -?

Decision:
1) It is necessary to calculate the mass of the resulting potassium chloride solution:
m solution (KCl) = m (KCl) + m (H2O) = 30 + 400 = 430 g;
2) The mass fraction of potassium chloride in the resulting solution should be calculated:
ω (KCl) = m (KCl) * 100% / m solution (KCl) = 30 * 100% / 430 = 6.98%.

Answer: The mass fraction of KCl is 6.98%.