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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: mandy on January 31, 2010, 04:05:11 pm

Title: Gravimetric analysis
Post by: mandy on January 31, 2010, 04:05:11 pm: Maxi-sweet tablets contain the artificial sweetener saccharin, C₇H₅NO₃S. During production, regular analysis of the tablets is carried out to ensure the quality of the product. The saccharin content per tablet is determined by gravimetric analysis. The S is oxidised to SO₄^2- ions, which are then precipitated using excess barium chloride, BaCl₂ solution.
a. Write an ionic equation for the precipitation of the sulfate ions.

During the analysis, 10 Maxi-sweet tablets were crushed and the S oxidised to SO₄^2- ions. This solution was then filtered and excess barium chloride solution was added to the filtrate. The precipitate formed was filtered out, washed, dried and weighed.
b. Why was the tablet solution filtered prior to the addition of barium chloride?

For part a, I don't know what to do. And part b, is the answer to ensure the tablet solution is pure or something :S
Title: Re: Gravimetric analysis
Post by: Edmund on January 31, 2010, 04:23:30 pm: a) $SO_4^{2-}(aq) + Ba^{2+}(aq) \rightarrow BaSO_4 (s)$

For ionic equations, you leave out the stuff that's not reacting (spectator ions), in this case they are the chlorine ions.

b) I think it is to remove anything that has not been oxidised to SO₄^2-
Title: Re: Gravimetric analysis
Post by: kyzoo on January 31, 2010, 04:23:45 pm: a.) SO₄^2-(aq) + Ba²⁺(aq) + 2Cl^-(aq) --> 2Cl^-(aq) + BaSO₄(s)
Note that the chloride ion is a spectator ion (does nothing)

Edit: This equation is wrong because it's not an ionic equation (because it includes spectator ions), take Edmund's one.

b.) I'm guessing that the tablet solution contained dissolved sulfate ions and solid crushed tablet powder. If this is the case, then the purpose would be to isolate the dissolved ions from the powder, so that the barium chloride solution only reacts with the sulfide ions, and not with any tablet powder. This enables accurate results.
Title: Re: Gravimetric analysis
Post by: mandy on January 31, 2010, 04:41:47 pm: Okay, thank you Edmund and kyzoo.

Now, I can't do part c.

c. It was found that 1.568g of barium sulfate precipitate was formed. Calculate the mass of saccharin present in the sample.

What I'm thinking is: write an equation of some sort with barium sulfate in it, then work out a ratio, then work out the mass of saccharin in relation to the barium sulfate. I'm not sure if this is right and what goes in the equation.
Title: Re: Gravimetric analysis
Post by: Edmund on January 31, 2010, 05:25:21 pm: Using the equation from part a),

$SO_4^{2-}(aq) + Ba^{2+}(aq) \rightarrow BaSO_4 (s)$

$n(BaSO_4)=\frac{1.568g}{233.4}=0.006718mol$

$n(SO_4^{2-})=n(BaSO_4)=0.006718mol$

Therefore, 0.006718mol of $SO_4^{2-}$ was reacted with Barium Chloride

The question initially stated S was oxidised to SO₄^2-. The equation can be written as:

$S(s) + 4H_2O(l) \rightarrow SO_4^{2-}(aq) + 8H^+(aq) + 6e^-$

So, $n(S)=n(SO_4^{2-})=0.006718mol$

$m(S)=0.006718 * 32.1 = 0.216g$

Hope I've not done something wrong here...
Title: Re: Gravimetric analysis
Post by: mandy on January 31, 2010, 05:31:03 pm: Edmund, you know how in the second last line, you multiplied 0.006718 by the Molar mass of sulfur? Wouldn't you multiply it by the Molar masss of saccharin, because thats what you're looking for? :S
Title: Re: Gravimetric analysis
Post by: evildoctorcow on January 31, 2010, 05:47:25 pm: Mandy, you're right. Multiply the number of moles by the molar mass of saccharin to give the mass of saccharin in the sample.

For b), the assumption is that barium chloride reacts only with the sulphate ions in solution, so that the precipitate is pure barium sulphate.
Title: Re: Gravimetric analysis
Post by: Edmund on January 31, 2010, 05:51:55 pm: Yeah, misread the question :P
Title: Re: Gravimetric analysis
Post by: mandy on January 31, 2010, 06:08:10 pm: Thanks guys ! I have two more parts to this question I'm not sure about.

f. What simple test could be performed to ensure that all sulfate ions were precipitated during the analysis?
Allow the same reaction to occur again and if the same precipitate (BaSO₄) is produced, then there are still sulfate ions that remain and weren't precipitated during the analysis.
g. What steps should have been taken to ensure that the precipitate contained no moisture?
Drying?

Are my answers correct?
Title: Re: Gravimetric analysis
Post by: kyzoo on January 31, 2010, 06:18:27 pm: They are correct but you need to clarify and elaborate

f) You would need to clarify what you mean by "the same reaction." But actually, I'm thinking that there would be a simpler method than this, just I don't know what it is.

g) How would you dry it?
Title: Re: Gravimetric analysis
Post by: Studyinghard on January 31, 2010, 06:19:27 pm: Quote from: kyzoo on January 31, 2010, 06:18:27 pm

g) How would you dry it?

chuck it in the oven:D
Title: Re: Gravimetric analysis
Post by: cindyy on January 31, 2010, 06:20:21 pm: Quote from: Studyinghard on January 31, 2010, 06:19:27 pm
Quote from: kyzoo on January 31, 2010, 06:18:27 pm

g) How would you dry it?

chuck it in the oven:D

at 210 degrees ;D
Title: Re: Gravimetric analysis
Post by: Studyinghard on January 31, 2010, 06:34:23 pm: Quote from: cindyy on January 31, 2010, 06:20:21 pm
Quote from: Studyinghard on January 31, 2010, 06:19:27 pm
Quote from: kyzoo on January 31, 2010, 06:18:27 pm

g) How would you dry it?

chuck it in the oven:D

at 210 degrees ;D

check the box for instructiosn ;P
Title: Re: Gravimetric analysis
Post by: mandy on January 31, 2010, 06:37:06 pm: LMAOO, thanks guys :D
Title: Re: Gravimetric analysis
Post by: Edmund on January 31, 2010, 06:55:24 pm: I would say something like weigh, then heat over a hot water bath, weigh and repeat the same process till weight does not decrease any further (meaning all the moisture has dried out)
Title: Re: Gravimetric analysis
Post by: mandy on January 31, 2010, 09:17:33 pm: Alright, thanks guys. :)
Title: Re: Gravimetric analysis
Post by: longy1991 on February 02, 2010, 12:04:23 pm: When you crush the maxi sweet tablets and make them up in solution, they WILL contain insoluble materials. As we end up weighing a precipitate, we can't have the weight of other insolubilities contributing to our precipitate mass. This would make our results very inaccurate. That's why you have to filter it :).
Title: Re: Gravimetric analysis
Post by: appianway on February 02, 2010, 07:37:10 pm: I think it also presumes that no other materials in the substance are soluble and react with the barium chloride to form a precipitate.
Title: Re: Gravimetric analysis
Post by: longy1991 on February 02, 2010, 11:05:07 pm: of course, coz many chlorides are insoluble and it is very unlikely that there would be no chlorides formed from the addition. However, that is not the question bro.
Title: Re: Gravimetric analysis
Post by: appianway on February 03, 2010, 06:53:12 pm: It's not the question, but it'd definitely add the the uncertainty of any calculations involving the amount of saccharin based on the mass of the precipitate as the other insoluble compounds would contribute to the mass of the precipitate.
Title: Re: Gravimetric analysis
Post by: longy1991 on February 04, 2010, 07:23:33 pm: it is unnecessary to point out the poor assumptions needed to carry out vce chemistry in a question lol. It's a waste of space unless specifically asked for.
Title: Re: Gravimetric analysis
Post by: appianway on February 04, 2010, 08:29:13 pm: It's unnecessary for this question, but it's probably better to at least be aware of what you're actually doing. I guess it has relevance if you're designing your own practical.
Title: Re: Gravimetric analysis
Post by: longy1991 on February 04, 2010, 10:48:12 pm: i agree.