What mass of sodium chloride should be taken to prepare 400 g of a solution with a mass fraction of salt equal to 20%?

We find the mass of sodium chloride in a given solution from the formula for determining the mass fraction of a substance.

W (in-va) = m (in-va) / m (solution) • 100%.

The mass of salt required to prepare the solution.

m (NaCl) = W (NaCl) • m (solution) / 100% = 20% • 400 g / 100% = 80 g.

Mass of water.

m (H2O) = m (solution) – m (in-va) = 400 g – 80 g = 320 g.

So you need to take 80 g of salt and dissolve in 320 g of water.