URGENT: interpreting gravimetric analysis experiments... (1 Viewer)

[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 :wave:

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

 

[Aerlinn]

Eeek, this is about as close we can get to the assessment, anything would be fab...!

To add onto the last post: Is the acetic acid used to dissolve all the Ca ions by forming Calcium acetate?

I was a little confused when I read up that CaSO4 was only slightly soluble. I know that all compounds are not completely soluble or insoluble anyway, but the fact that CaSO4 is mentioned specially as slightly soluble probably means it is very true for this case. Does that mean that not all the Ca ions from the solution are precipitated? Wouldn't that produce a lowered/inaccurate result? (ie. why would we want to produce a precipitate that's not insoluble?) Not sure whether anyone knows...

You see, I came up with these successive reactions for this experiment, I'm pretty sure they're all right, but they just don't connect:

First, CH3COOH(aq) + CaCO3 (s) ----> H2CO3(aq) + Ca(CH3COO)2
which then becomes: CH3COOH(aq) + CaCO3 (s) ----> H2O(l) + CO2(g) + Ca(CH3COO)2 (aq)
Then there is: H2SO4(aq) + CaCO3(s) ---> CaSO4(s) + CO2(g) + H2O(l)
or: H2SO4(aq) + CaCO3(aq) ----> CaSO4(aq) + H2O(l) + CO2 (g)
(Both of these are valid, yes?)

My dilemma is this: H2O(l) and CO2(g) and Ca(CH3COO)2 (aq) form first. The CO2 probably escapes into the air (if not, correct me ), so you're left with H2O and Ca(CH3COO)2 in solution. We next add Sulphuric acid, H2SO4. There is no CaCO3 for it to react with! Instead, there is Ca(CH3COO)2, so the SO4 ions would react with the Ca ions to form the precipitate, CaSO4, is that right?
So if I'm right at all, this reaction: H2SO4(aq) + CaCO3(aq) ----> CaSO4(aq) + H2O(l) + CO2 (g) (or the other form, whichever will do) never occurs. Instead, we should probably just write the ionic equation: Ca(2+) (aq) + SO4(2-) (aq) ----> CaSO4 (s) ... Am I right? I need clarification...


Also, I came up with these possible 'sources of error' with the fact that CaSO4 was partially soluble in mind:
-For the experiment, we assume that all the Ca ions are precipitated as CaSO4 (since CaSO4 is slightly solube). (?)
-Assume no other precipitates form with the SO4 ions apart from the Ca ions.
-Assume all CaCO3 dissolves
-Assume that all water is removed from the precipitate
-Assume there are no impurities on the surface of the precipitate (?)
-Loss of precipitate during filtering, transfers or if precipitate's attached to any of the apparatus.
Ok, I'm not entirely sure all of them/ any of them are valid, can anyone correct me on them? Also, if you know of any other typical gravimetric analysis errors, or questions that might be asked about one of these experiments, that would be pretty helpful

Really hopeful someone can help me with this today! I'm going to be doing it tomorrow.

 

[xiao1985]

hope it's not too late...


1) for step 3, just in case in the tablet, there are small rocks which fall in accidentally, you don't want to mistake those rocks with Ca ions... that's why you want to filter the mixture after you added acetic acid (note, acetic acid will not dissolve rocks, but will dissolve ca ions if present)

2) examine the residue - i dont' know... maybe if the appearance of the residue is a bit... dodgy, you can assess the situation and make a better decision?! (for eg, if alot of things ppt'ed, you might want to check the stuff you used is acetic acid... you might have mistaken hydrochloric acid or sulfuric acid with it... or perhaps wrong label has being attached to it...)

3) well, H2SO4 ain't terribly expensive... but note that afterwards, you need to discard the waste... if you have 20 L of 2M H2SO4, that's not the most pleasant thing in the world... that's why in chemistry, you always want to use the minimum amount of reagent/solvent possible.

4) just to make sure ALL and i mean ABSOLUTELY ALL ppt got rinsed out and into the filter paper

5) you are absolutely, totally, 100% right... here, have a cookie

6) to be exactly, it should be:
CaCO3 + 2H+ <----> Ca 2+ + H2O + CO2
Ca 2+ + SO4 2- <----> CaSO4 (s)
but then, it's just me... and yes for the mole ratio question... though i doubt you will need to concern yourself with that

7) you dont' want to add too much water if the stuff is sparingly soluable... also if you want to do this VERY VERY accruately, you want to leave your sample in dessicator or oven to dry... not just in air

8) calculations, sources of error




re: 2nd post

- reaction path way: see above... it dissolve CaCO3 by in fact eating away the CO3 2- ions...

- you are absolutely right in saying that i might not be extremely accurate... especially in this case, that ppt formed is sparingly soluable...

- "My delimma is ... " again, you are absolutely right... i suddenly have the urge to pop the question: "why are you so smart?!"

- list of errors - i think you are fine with all these sources of errors... just you need to know how to improve your expt knowing sources of errors (ie, leave in oven to take away all water etc )

 

[Aerlinn]

Thanx for the help ^^ heheh, have a cherry *hands you one*
Out of curiosity--

you are absolutely right in saying that i might not be extremely accurate... especially in this case, that ppt formed is sparingly soluable

you dont' want to add too much water if the stuff is sparingly soluable

*puzzled look*

 

[xiao1985]

you are absolutely right in saying that i might not be extremely accurate... especially in this case, that ppt formed is sparingly soluable

saying that IT*
beg your pardon

you dont' want to add too much water if the stuff is sparingly soluable

the amount of the stuff dissolved depends on how much water (solvent) you add... the more solvent, the more of it will dissolve (think this way: if you have 1 L of water, 2 g of CaSO4 will dissolve, which means if you add 10L of water, then 20 g of your precious ppt will dissolve and cannot be captured by filtration)

 

[Aerlinn]

Ah... ok, never thought of that before. So the main thing is, dont use too much distilled water. I wonder why she didn't warn us about that.

I also wondered why our teacher was all, 'Don't use more than 1mL acetic acid!!' when we were rinsing flask and paper with it today (step 3) Do you have any idea?

Oh, and you know how you said

just you need to know how to improve your expt knowing sources of errors

My suggested improvements, (apart from following the prac and being accurate, which I think I did)...
-For the experiment, we assume that all the Ca ions are precipitated as CaSO4 (since CaSO4 is slightly soluble).
To make sure, add many aliquots of H2SO4?
-Assume no other precipitates form with the SO4 ions apart from the Ca ions.
I ain't too sure how to improve this, 'cause in another Grav. analysis thingy we did, we lowered the pH by adding HCl (it involved getting a ppt of BaSO4) to make sure no other ions formed a ppt with the Ba(2+). Don't know if there's a similar thing that can be done for this one, or maybe something else... help =S
-Assume all CaCO3 dissolves
Make sure it all does by taking care to make sure reaction goes to completion, add acetic acid to dissolve all the little bits?
-Assume that all water is removed from the precipitate
Make sure it happens by weighing to constant mass, except with the CaSO4.2H2O ppt, I was informed you can't drive away all the water 'cause there is that H20 in the crystals...?
-Assume there are no impurities on the surface of the precipitate
Hm, if you notice impurities, how would you get rid of em? *confuzzled*
-Loss of precipitate during filtering, transfers or if precipitate's attached to any of the apparatus.
Make sure you don't lose any?

Ok, so with my rough attempt at writing out some improvements, do you have any possible suggestions?

(ek, write-up tomorrow )

 

[xiao1985]

1) don't add too many... just 1 or 2 aquilots will be sufficient... too much solvent you are risking dissolving a proportion of CaSO4

2)you can do similar things... since CaCl2 is also soluable but chloride of alot of other things are non soluable

3) yes

4) water in the crystal ain't really all that of a problem... you account for that during your calculations... perhaps thinking of storing it in the dessicator overnight before weighing is a good idea...

5) you can't.... weight them as soon as they are ready, and make sure your buddy next door ain't doing his lawns

6) you can't either... no matter how precise you are, you will eventually lose abit here and there... you just have to say "i lost minimal amount by doing it very precisely" or s th like that... but then, i won't even include how to deal with the last one... abit like human error, you can't do abot about it...

 

[Aerlinn]

Okey, thnx In the write-up, there was a thing that stumped me. It asked us what other substances could have been in the tablet. They didn't seriously want us to list the ingredients did they?!

 

[xiao1985]

not really, but it's inevitable that there WILL be impurities in the tablet... things like rocks, ... uhm... other rocks etc...

and if they are present in the tablet, they have the potential to ruin your result, as the ppt you weighted out maybe not be as accurately as possible.