Heating FeSO4.7H2O: Reasons for Different Values of x

[coconut62]

Yesterday I had a practical examination, and one question involves the heating of FeSO4.xH20 to get rid of the water of crystallization to determine the value of x.

A crucible was used to contain the crystals. Just a few grams.

I had found the value of x to be 7. And now a part of the question says, a student did the same experiment and obtained a value of 9. What could be the reason for this?

I wrote that because during heating some of the FeSO4 crystals "jump" out from the crucible, resulting in a larger mass loss and the mass lost is assume to be the mass of water of crystallization.

Is that acceptable?

 

[NascentOxygen]

That sounds plausible, yes. I vaguely recall that iron sulphate crystals do crackle as they lose water when heated, so I would expect this may splatter some powder out of the crucible if you were to leave its lid off.

But careless lab technique may not be quite the answer your examiner was looking for. Perhaps he was hoping for ... http://answers.yahoo.com/question/index?qid=20110908041238AAw5ka4

 

[Borek]

Worth of noting here that exact composition of many hydrates depends on the temperature, humidity and sample history (where and how it was stored for the last few hours).

Also, FeSO₄ is not a stable salt - iron gets oxidized by the air oxygen to Fe(III), so what you think is FeSO₄·7H₂O can be in fact a mixture of different sulfates.