[MitchTwitchita]

I just finished a lab and I was wondering if anyone could point out where I'm going wrong.

Here are my findings for CaSO4:

                                                        After First Heating  After Second heating

Wt. of crucible, lid, and hydrate       35.0537g                34.9779g
Wt. of dry crucible and lid                34.0226g                34.0226g
Wt. of hydrate                                   1.0331g                  0.9553g

Wt. of crucible, lid, and anhydrate   34.9779g                34.9781g
Wt. of crucible and lid                      34.0226g                34.0226g
Wt. of anhydrate                               0.9553g                  0.9555g


Questions:

1.  Calculate the % by weight of water in your unknown hydrate.  Report the % to four digits.

1.0331g CaSO4*H2O - 0.9555g CaSO4 = 0.0756 g H2O
0.0756g/1.0311g x 100% = 7.332%

2.  Using the % water from Part 1 above, calculate the moles of water.  (Hint:  Assume a 100g sample of hydrate.)  Report to four digits.

7.332g H2O x 1mol. H20/18.02 g H2O = 0.6806 mol H2O

3.  Using the & anhydrate, calculate the moles of anhydrate.  Report four digits.

100% - 7.332% = 92.668%
92.668g CaSO4 x 1 mol CaSO4/136.15g CaSO4 = 0.6806 mol CaSO4

4.  Using the anhydrate formula (CaSO4) given to you in lab, and the values from Parts 2 and 3 above, give the formula of your unknown hydrate.  Show your calculation.  Round the water to the nearest 1/2 or whole number of moles.

H2O = 0.4069 mol/0.4609 mol =1
CaSO4 = 0.6806 mol/0.4609 mol = 1.673
x 3 = 5CaSO4*3H2O

These calculations don't seem to be right, can aybody please help me understand?

[Dan]

There is a typo in part 2, your moles of water should be 0.4069 (which you used in part 4, so you haven't carried the error through).

As far as I can see you have not made any mistakes in your calculations. A quick wiki suggests that you had plaster of paris to start with:

The partially dehydrated mineral is called calcium sulfate hemihydrate or calcined gypsum (commonly known as plaster of Paris) (CaSO4·nH2O), where n is in the range 0.5 to 0.8.

Your "n" is 0.6, so you probably haven't done anything wrong.

[MitchTwitchita]

Thanks a bunch Dan!  5CaSO4*3H2O just didn't seem right.