[jgillespie]

Doing a basic determine the formula of a hydrate lab using copper sulfate pentahydrate recrystallized from RootKill.  When I divide the molecular weight of the difference-between-anhydrate-and-hydrate by molecular weight of hydrate I get six not five.

Searched earlier Chemical Forums posts and found pretty good how to's but nobody that has run into this problem (did I miss something?).

I imagine this is the result of an error common among novices.  Can anyone enlighten me? 

I have run through the procedure three times, used two different crucibles, and have heat-dried the crucible before weighing the hydrate. Consistently getting the same results - 6, not the expected 5.

[AWK]

Ammonium copper sulfate hexahydrate is known, cobalt sulfate hexahydrate also exists. Check your reagent, and show us your calculations.

[jgillespie]

I just took RootKill (99/9% copper sulfate pentahydrate), that I recrystallized by boiling in water then rapidly cooled in an ice bath, and put it in a glass crucible (then again in a stainless steel crucible) and heated it up till it went from all blue to all gray. (If anyone has a copy of Robert Bruce Thompson's Illustrated Guide to Home Chemistry Experiments, please see Laboratory 6.6)

Mass of glass crucible                 73.87 g
Crucible + hydrate                      79.31
mass of hydrate                          5.44

Crucible + anhydrate                   77.09
Mass of anhydrate                        3.22

mass of water of crystallization      2.22

mass of anhydrate / formula weight of hydrate = 3.22 / 159.6 = 0.021
mass of water/ formula weight of water          = 2.22 / 18.05 = 0.123

0.123 / 0.021 = 5.85


Mass of steel crucible                 77 g
Crucible + hydrate                      83.39
mass of hydrate                          6.39

Crucible + anhydrate                   80.80
Mass of anhydrate                        3.8

mass of water of crystallization      2.59

mass of anhydrate / formula weight of hydrate = 3.8/ 159.6 = 0.024
mass of water/ formula weight of water          = 2.59 / 18.05 = 0.144

0.144 / 0.024 = 6

I repeated the lab a third time after more carefully drying the steel crucible and got very similar results.

[AWK]

Calculations are correct. The only explanation - crystals were wet.

[jgillespie]

Yes, they were still wet in the jar where I stored them after recrystallization.  That must be it - I will verify.  Thank you!