[helpme123]

A student was working to find out the formula of magnesium sulfate hydrate and found that there were 10 moles of water per mole of magnesium sulfate. The real value is 7 moles of water per mole of magnesium sulfate. What experimental error might have led to this discrepancy? Be specific and support your answer with CALCULATIONS if necessary.

I thought that when he weighed the magnesium sulfate that would be too much, but the law of definite proportions denies this. I also thought when he weighed the magnesium sulfate, some could have dropped out, but that would have led to a drop in the weight, not increase.

[fledarmus]

Do you know what the student did to try to determine the amount of water? (experimental procedure)

[doc_ross]

Given that the error was in his experimental procedure, it seems likely that he failed to dry his crystals to the extent necessary to ensure that hydration was limited to the crystal lattice and not simply contaminating water within the crystal powder.
That is; water of hydration consists of water molecules integrated into the crystal lattice.  Water contamination is water stuck to the crystal surfaces.  If he did not dry his crystals under appropriate conditions, he may have water stuck to the crystals.  Careful though, heating the crystals can drive water out of the crystal lattice and result in partial or complete dehydration.  They must be heated at the appropriate temperature or allowed to equilibrate with moisture in the atmosphere.