How will the mass fraction of a substance change if 20 g of a substance is added to 100 g of a 20% solution.

Given:

m (solution) = 100 g

w% (salt) = 20%

m (salt) = 20 g

To find:

w% (salts in the final solution) -?

Decision:

1) Find the mass of salt in the primary solution:

m (salt) = 100 g * 0.2 = 20 g;

2) Find the mass of salt after adding a portion:

m (salt) = 20 g + 20 g = 40 g;

3) Find the mass of the solution after adding salt:

m (solution) = 100 g + 20 g = 120 g;

4) Find the mass fraction of the substance in the resulting solution:

w% (things) = m (salt): m (solution) * 100% = 40 g: 120 g * 100% = 33.3%;

5) Find the difference in the concentration of solutions:

Rev. w% (salt) = 33.3% – 20% = 13.3%.

Answer: the concentration of the substance changed by 13.3%.