Determine the mass of water that needs to be added to 30 grams of a 40% solution to make the solution 10%.

1. Let’s calculate the mass of salt in solution:

m (salt) = w (salt) * m1 (solution) = 0.4 * 30 = 12 g;

2. Let the mass of the added water be x, we find the mass of the diluted solution:

m2 (solution) = m1 (solution) + m (H2O) = 30 + x g;

3.We express the mass fraction of salt after dilution:

w2 (salt) = m (salt): m2 (solution);

0.1 = 12: (30 + x);

3 + 0.1x = 12;

0.1x = 4;

x = 40 g.

Answer: 40 g.