Table 3: Determine the percent water in the sample

 

Parameter	Trial 1
Mass of crucible, lid, and hydrated salt (g)	39.65 g
Mass of crucible, lid, and anhydrous salt (g)	38.78 g
Mass of crucible and lid (g)	37.76 g
Mass of hydrated salt (g)	1.89 g
Mass of anhydrous salt (g)	1.02 g
Mass of water lost (g)	0.87 g
Percent of water in the hydrated salt (%)	46.03 %
Number of moles of anhydrous salt (mol)	0.0064 mol
Number of moles of water lost (mol)	0.0483 mol
Ratio between salt and water	1:8
Formula of hydrated salt	CuSO4 (s) + H2O (g)

If the results do not yield a molar ratio of whole numbers for copper sulfate and water, it is justified to round the numbers to the nearest whole numbers because to record the formula of hydrated salt, you use the smallest whole number ratio.  7.56 moles is rounded to 8 moles of water because you already have over 7 moles, so you are considering the amount of substance already in the crucible.

 

If the dehydrated sample was left exposed to air, it would introduce a significant error because the oxygen entering the substance would alter the result molar ratio of hydrated salt to water.

 

The crucible must cool to room temperature before taking any mass measurement because if not, the crucible could damage the scale.

 

If the crucible was heated too rapidly and some sample material spattered out, the final percent water calculations would be smaller than they should have been, calculated with an accurate original amount of material before heating took place.

 

If the lid was removed from the crucible while the sample was  being heated and a cold watch glass was held over it, the particles wouldn't completely dehydrate because of the extra moisture they were exposed to from the air.