mH2O = 390 (g)
\(n_{CuSO_4.5H_2O}=\dfrac{x}{250}\left(mol\right)\)
=> \(n_{CuSO_4}=\dfrac{x}{250}\left(mol\right)\)
\(m_{CuSO_4}=\dfrac{x}{250}.160=0,64x\left(g\right)\)
mdd sau pư = x + 390 (g)
=> \(C\%=\dfrac{0,64x}{x+390}.100\%=7,27\%\)
=> x = 50 (g)