To 150 g of a 10% solution was added 50 g of water. Find the mass fraction of salt in the solution after dilution.

η1 = 10%.

mр1 = 150 g.

md = 50 g.

η2 -?

Let’s find how much water mw1 and how much salt mc was in the original solution.
The mass of the entire solution mр1 = 150 g is 100%. Since the salts mс in this solution are 10%, then mс = mр1 * η1 / 100%.

mс = 150 g * 10% / 100% = 15 g.

When water was added to the solution, the total mass of the solution became mр2 = mр1 + md.
mr2 = 150 g + 50 g = 200 g.

The mass of the solution mр2 = 200 g, after adding water it is 100%, and the salt in it remains mс = 15 g.
η2 = mс * 100% / mр2.

η2 = 15 g * 100% / 200 g = 7.5%.

Answer: the mass fraction of salt in the solution after dilution will be η2 = 7.5%.