To 250 g of a solution with a mass fraction of a substance in it of 20%

To 250 g of a solution with a mass fraction of a substance in it of 20%, add 20 g of water. Determine the mass fraction of the solute in the new solution.

1. Let’s find the mass of the substance in the first solution.

W = m (substance): m (solution) × 100%,

hence m (substance) = (m (solution) × w): 100%.

m (substance) = (250 g × 20%): 100% = 50 g.

2. Find the mass of the resulting solution after adding 20 g of water to it.

m (solution) 2 = m (water) + m (substance).

m (solution) 2 = 250 g + 20 g = 270 g.

3. Let’s find the mass fraction of the substance in the resulting solution.

W = (50g: 270g) × 100% = 18.52%.

Answer: 18.52%.