To 80 grams of a 10% solution, 20 milliliters of water was added, which was the mass fraction of the substance.

The mass fraction of a solute is the ratio of the mass of a solute to the mass of a solution, expressed as a percentage.

Knowing the mass fraction, we find the mass of the solute in the original solution.

m (in-in) = 80 * 10/100 = 8 g

After adding 20 milliliters of water, the mass of the solution increased by 20 g, since the density of water is 1 g / ml.

Let’s calculate the mass of the resulting solution:

m (solution) = 80 + 20 = 100 g.

Now we can find the mass fraction of the solute:

w = m (in-in) / m (r-r) * 100% = 8/100 * 100% = 8%