How much H2O do you need to take to prepare 120g of 10% NaCL solution?

Given:
m solution (NaCl) = 120 g
ω (NaCl) = 10%

To find:
m (H2O) -?

Solution:
1) Calculate the mass of sodium chloride in the solution:
m (NaCl) = ω (NaCl) * m solution (NaCl) / 100% = 10% * 120/100% = 12 g;
2) Calculate the mass of water that will be needed to prepare the solution:
m (H2O) = m solution (NaCl) – m (NaCl) = 120 – 12 = 108 g.

Answer: The mass of H2O is 108 g.