40 g of water was added to a sodium hydroxide solution weighing 120 g and a mass fraction equal to 6%.

40 g of water was added to a sodium hydroxide solution weighing 120 g and a mass fraction equal to 6%. Determine the mass fraction of sodium hydroxide in the resulting solution.

Given:

m (solution) = 120 g.

W1 = 6%,

m (water) = 40 g.

W2 -?

Decision:

Find the mass of sodium hydroxide dissolved in the solution.

Mass fraction of a substance is calculated by the formula:

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

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

m (substance) = (120 g × 6%): 100% = 7.2 g.

m (solution) 2 = 120 g + 40 g = 160 g.

W = (7.2g: 160g) × 100% = 4.5%.

Answer: 4.5%.