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Topic: Problem of the week - 25/06/2012  (Read 8440 times)

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Offline Borek

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Problem of the week - 25/06/2012
« on: June 25, 2012, 06:29:52 AM »
You have inherited an old cabinet containing reagent bottles. As buying reagents is becoming difficult these days, you want to keep as many reagents as possible, but to do so you have to know the content of the bottles. Unfortunately, many of the labels are either stained or have fallen off. You are limited with tests that you can do, but you do have some lab glass, reagents and an old pH meter.

Label on one of the bottles is hand written and badly stained and the only readable information is that it contains a 98% solution (with an exclamation mark after 98%, probably to mark the substance as dangerous) of something with molar mass of 98. Content of the bottle is a clear, syrupy liquid. After transferring 1 mL of the liquid to the beaker mass of the beaker increased by 1.8 g, suggesting density around 1.8 g/mL. After adding 100 mL of water pH of the solution was around 1.50, so apparently it is a strong acid. Titration of 10 mL of the diluted solution with 0.10 M NaOH against phenolphthalein required 37 mL of the titrant, which suggests acid is diprotic. Putting a drop of the concentrated acid into solution of the calcium chloride yields a white precipitate. It all suggests it is a sulfuric acid... but can you be sure?
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Offline Rutherford

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Re: Problem of the week - 25/06/2012
« Reply #1 on: June 25, 2012, 07:14:01 AM »
Maybe H3PO4?
Mass is good, density is good, with NaOH it reacts only till all HPO4- is formed (like it is diprotic), precipitate forms with Ca2+.

Offline AWK

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Re: Problem of the week - 25/06/2012
« Reply #2 on: June 25, 2012, 08:59:06 AM »
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so apparently it is a strong acid
?
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Offline stewie griffin

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Re: Problem of the week - 25/06/2012
« Reply #3 on: June 25, 2012, 09:51:06 AM »
Test it's reaction with copper metal. If you get blue solution (copper sulfate), you've got sulfuric acid.

Offline discodermolide

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Re: Problem of the week - 25/06/2012
« Reply #4 on: June 25, 2012, 11:28:43 AM »
I agree it's H3PO4.
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Offline stewie griffin

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Re: Problem of the week - 25/06/2012
« Reply #5 on: June 25, 2012, 11:39:53 AM »
Concentrated phosphoric is typically an 85% solution. Above that concentration I believe you have crystalline phosphoric acid. The problem says that we have a liquid, not a mix of liquid and solid.

Offline Borek

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Re: Problem of the week - 25/06/2012
« Reply #6 on: June 25, 2012, 12:35:53 PM »
Concentrated phosphoric is typically an 85% solution. Above that concentration I believe you have crystalline phosphoric acid. The problem says that we have a liquid, not a mix of liquid and solid.

This is sad, looks like sources I considered reliable are not always reliable.

Yes, the idea was that the correct answer is that it is a phosphoric acid (which can be checked by estimating expected pH of the solution), but the wording was intentionally misleading to "prove" it is a sulfuric acid. 98% is not a typical concentration as stock solutions are usually 85%, so the exclamation mark after the concentration was to mark non-typical concentration, not a dangerous substance. pH of 1.5 for 0.18M solution is low, but not THAT low to suggest a really strong acid. When titrated they both usually behave like diprotic, they have almost identical molar masses, they both - when pure - look very similar. Finally, if you trust International Critical Data Tables like I did, at room temperature they have almost identical density. No idea why IDT lists these numbers the way it does (note that it doesn't list densities for 10°C).

Another elaborate construction down the drain  :P
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Offline AWK

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Re: Problem of the week - 25/06/2012
« Reply #7 on: June 26, 2012, 04:51:39 AM »
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pH of 1.5 for 0.18M solution is low, but not THAT low to suggest a really strong acid
Strong acid with concentration close to 0.2 M shows pH closer to 0.5 than to 1.5.
AWK

Offline Borek

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Re: Problem of the week - 25/06/2012
« Reply #8 on: June 26, 2012, 05:04:43 AM »
Strong acid with concentration close to 0.2 M shows pH closer to 0.5 than to 1.5.

Sigh.

Have you read my comment about the wording being INTENTIONALLY MISLEADING?
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