How many grams of Na2SO4 should be dissolved in 400 grams of water to obtain an 8% solution?

Given:
m (H2O) = 400 g
ω (Na2SO4) = 8%

To find:
m (Na2SO4) -?

Decision:
1) Let m (Na2SO4) = (x) r;
2) Calculate the mass of the Na2SO4 solution:
m solution (Na2SO4) = m (Na2SO4) + m (H2O) = (x + 400) g;
3) Calculate the mass of the Na2SO4 solution:
m solution (Na2SO4) = m (Na2SO4) * 100% / ω (Na2SO4) = x * 100% / 8% = (12.5x) g;
4) Make an equation and solve it:
m solution (Na2SO4) = m solution (Na2SO4);
x + 400 = 12.5x;
11.5x = 400
x = 34.8;
5) Determine the mass of Na2SO4:
m (Na2SO4) = x = 34.8 g.

Answer: The mass of Na2SO4 is 34.8 g.