When 2.5g of copper sulphate are dissolved in 500ml of water, the concentration of the solution is

V(H₂O)=500 mL
p(H₂O)=0.9982 g/mL

m(H₂O)=V(H₂O)p(H₂O)

mass of salt solution:
m=m(H₂O)+m(CuSO₄)=V(H₂O)p(H₂O)+m(CuSO₄)

w = m(CuSO₄)/m

w=m(CuSO₄)/{V(H₂O)p(H₂O)+m(CuSO₄)}

w=2.5/{500*0.9982+2.5}=0.004984  (0.4984%)