What mass of sodium sulfate decahydrate needs to be dissolved in 800 g of water to obtain a 10% salt solution?

Given:
m (H2O) = 800g
ω (salt) = 10% = 0.1
To find:
m (salt) -?
Decision.
Sodium sulfate decahydrate is a crystalline hydrate Na2SO4 • 10H2O
Μ (NaSO4 • 10H2O) = 322 g / mol
M (Na2SO4) = 142 g / mol
Let you need X mol of sulfate decahydrate, its mass is 322X g, sodium sulfate in it is 142X g.
ω = m (in-va) / (m (solution) + m (crystalline hydrate)
0.1 = 142X / (800 + 322X)
1420X = 800 + 322X
1098X = 800
X = 0.729 mol
Sulfate decahydrate mass:
0.729 * 322 = 234.738 g
Answer: 234.738g.