URGENT: interpreting gravimetric analysis experiments...

[Aerlinn]

I have this experiment, but I haven't done it yet. It's a gravimetric analysis one where we want to determine the percentage of calcium in a calcium diet supplement tablet.
The procedure goes as follows:
1. Weight one Caltrate tablet accurately into a clean 100ml conical flask
2. Add 50.0mL of the 2M acetic acid and allow the reaction to proceed until the tablet is digested (approx. 30 minutes)
3. Using a Buchner funnel, filter the mixture carefully, rinsing the flask and paper with 2M acetic acid.
4. Examine and record the residue on the filter paper and discard.
5. To the filtrate, add the 2M H2SO4 10mL aliquots until no further precipitate forms. Allow to stand to effect the complete reaction.
6. Weigh accurately a clean, dry filter paper.
7. Filter the precipitate, rinsing the flask with distilled water.
8. Transfer the filter paper with the CaSO4.2H2O to a previously weighed evaporating basin.
9. Dry to constant mass using oven at 100C.

Ok, I have a little bit of trouble interpreting this...
1. For Step 3, why exactly is it that we always need to filter the mixture to get a filtrate? Why does it say to rinse the flask and paper with 2M acetic acid? (I assume the flask it refers to is the one we are supposed to filter the solution into...)
2. For Step 4, why should we 'examine and record the residue'? Should there be anything on there worthy of interest

3. For step 5, is the 2M H2SO4 added in only 10mL aliquots for a reason? ie. Why is the whole thing not tipped in
4. For step 7, it says to rinse the flask with distilled water (after the 2nd filtration). I don't see why this should make a difference... can anyone tell me?
5. For step 8, we are supposed to previously weigh the evaporating basin. Is that because when we finally weigh the evaporating basin + filter paper + the precipitate of CaSO4.2h2O after doing step 9, we would subtract the mass of the evaporating basin and filter paper to find the mass of the precipitate? Correct me if I'm wrong...
6. I worked out the reaction in the experiment to be:
H2SO4(aq) + CaCO3(aq) ----> CaSO4(s) + H2O(l) + CO2 (g)
Not sure if it's right... Because... since the precipitate comes out as CaSO4.2H2O, wouldn't it make more sense if the mole ratio between the CaSO4 and the H2O were 1:2 ?
7. Any errors you can see from the experiment. I have some, but wondering whether there are more...
8. Hmmm... if you've done a gravimetric analysis experiment before, are there any typical questions that might be asked of you with these types of experiments?

A great bit of confusion... Would be happy if someone could help me out here

PS. Really urgent, so any insight at all really soon would be really good![/b]

[RobJim]

1. Steps 1 and 2 will dissolve many of the components of the tablet, including the calcium, but will leave some insoluble residue. Filtering this residue out will remove part of the unwanted part of the tablet. It's part of the process of getting the calcium separate from the rest of the tablet. You rinse the flask and paper to make sure you got all the calcium. Notice the rinse is using the same acid that was used to dissolve the tablet. The flask you're rinsing is the one you dissolved the tablet in, not the one that the filtrate goes into.

2. No, you just want to know the mass and appearance of this part of the tablet for completeness. There's nothing interesting there.

3. You add the acid in specific amounts so you can calculate exactly how many moles of acid were used, I think. This will allow you to determine how many moles of CaSO4 are produced, which tells you how much calcium there is.

4. Again, the rinse is to make sure you don't lose any material. In this case you're trying to avoid losing any precipitate.

5. Correct.

6. The reaction looks right at first glance. The water that complexes with the product is not part of the chemical reaction but comes from the solution. So, you produce a little water chemically, but the CaSO4 product afterwards complexes with any water around, and most of that won't be the water produced in the reaction.

7. I don't see anything.

8. Don't know.