Analysis of result from CuSO4 stiochmetry lab

[MayaL]

In a lab we recently did we were supposed to find the empirical formula for CuSO₄. We've just started doing mole calculations and I am not sure I got it yet. We heated the hydrate to vaporise the water.

CuSO₄.xH₂O (s) → CuSO₄ (s) + xH₂O (g)

Initial mass =0.983g

After 15 min of heating= 0.515g

0.983g-0.515g=0.468g water lost

No. of moles of H₂O lost=
0.468/[2(1.01) + 16.00] =
0.468/18.02 = 0,026 mole

No. of moles of CuSO₄ =
0.515/[63.55 + 32.06 + 4(16.00)]
0.515/159.61= 0,0032 mole

No. of moles of H₂O lost for every mole of CuSO₄=
0,026/0,0032 = 8.125

Indicates that x=8 CuSO₄.8H₂O (s) → CuSO₄ (s) + 8H₂O (g)

Why didn't I end up with CuSO₄.5H₂O (s) → CuSO₄ (s) + 5H₂O (g) as expected? Have I done something wrong in my calculations? If we didn't heat the hydrate long enough, how would this affect the result? What other factors could affect the result?

Thanks