How much water should be taken to prepare a solution weighing 30 g with a mass fraction of a substance of 5%.

Given:
m solution (substance) = 30 g
ω (substances) = 5%

To find:
m (H2O) -?

Decision:
1) Calculate the mass of the substance in the solution:
m (substances) = ω (substances) * m solution (substances) / 100% = 5% * 30/100% = 1.5 g;
2) Calculate the mass of water that will be needed to prepare the solution:
m (H2O) = m solution (substance) – m (substance) = 30 – 1.5 = 28.5 g.

Answer: The mass of H2O is 28.5 g.