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Topic: Identify a substance  (Read 285 times)

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Offline meatlover

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Identify a substance
« on: March 18, 2020, 04:35:12 AM »
Hi, I am having some issues with a pre-exam question in general chemistry.

The following information is given about a substance:
  • It is a white powder
  • It has the molar mass of 150g/mol
  • It's melting point is 165 degrees C
  • To neutralise half a mole of the substance you require 1 mol of NaOH
  • Combustion of the substance gives, among others, 4mol of CO2
  • Optical activity: 0 degrees
  • Solubility: 1,25kg / Litre of water
  • It does not contain any nitrogen atoms or halogens
"Suggest a structure using this information and motivate clearly."

Using the combustion I can figure out that it must contain 4 carbons (assuming total combustion), but I am unsure as to how to proceed.

Any and all help would be greatly appreciated :)

Offline AWK

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Re: Identify a substance
« Reply #1 on: March 18, 2020, 04:50:19 AM »
You have acid with four carbon atoms and MW 150. Try to fit the number of oxygen and hydrogen atoms to this mass.
AWK

Offline meatlover

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Re: Identify a substance
« Reply #2 on: March 18, 2020, 04:55:38 AM »
You have acid with four carbon atoms and MW 150. Try to fit the number of oxygen and hydrogen atoms to this mass.

Thank you for your quick answer :).

I thought about doing that, but would it not be possible for Sulphur to be present in the substance? That would complicate the calculations quite a bit and open many possibilities.

Offline AWK

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Re: Identify a substance
« Reply #3 on: March 18, 2020, 07:21:00 AM »
You multiply problems. Start with the simplest compounds that only contain hydrogen and oxygen. You can then try to replace two oxygen atoms with sulfur and check your physical properties. By the way, I think that one of the properties - melting point and solubility is wrongly assigned from two different isomers.
AWK

Offline meatlover

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Re: Identify a substance
« Reply #4 on: March 18, 2020, 08:02:03 AM »
You multiply problems. Start with the simplest compounds that only contain hydrogen and oxygen. You can then try to replace two oxygen atoms with sulfur and check your physical properties. By the way, I think that one of the properties - melting point and solubility is wrongly assigned from two different isomers.

Thank you once more :).

I have been able to narrow it down to C4H6S3, and since the optical activity is 0 that must mean that there is an equal amount of S and R centres, other than that I am unsure what else I can determine.

Regarding the solubility/melting point I will have to discuss it with my professor, and I will be able to ask any last questions.

Offline AWK

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Re: Identify a substance
« Reply #5 on: March 18, 2020, 08:03:58 AM »
Neglect sulfur at the beginning. Most common organic compounds contain C, H, and O
AWK

Offline MNIO

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Re: Identify a substance
« Reply #6 on: March 21, 2020, 06:27:19 PM »
I think you're just guessing based on the molar mass.

take the clues one at a time
   It is a white powder
       <probably doesn't contain S since it's white powder right?>
   It has the molar mass of 150g/mol
   It's melting point is 165 degrees C
       <not a salt & probably doesn't contain any metals>
   To neutralise half a mole of the substance you require 1 mol of NaOH
       < take that as meaning it has 2 acid H's per molecule>
   Combustion of the substance gives, among others, 4mol of CO2
       <take that as meaning 4 C's per molecule>
   Optical activity: 0 degrees
      <take that as meaning you either (1) have a racemic mixture or (2) don't have
        an optically active molecule>

   Solubility: 1,25kg / Litre of water
   It does not contain any nitrogen atoms or halogens

**********
so let's start here. 

what molecules have 4 carbons and 2 acid H's?  this help?
  https://en.wikipedia.org/wiki/Dicarboxylic_acid
or some variation of that 4 carbon dicarboxylic acid?

notice the molar mass is 32g/mol short of the 150 you're given?
would 2 oxygen atoms make the difference?

how would you fit 2 additional O atoms in that molecule?
what would it be called?
what are the possible isomers?
what are the possible optical activities of such a molecule?
do any of those match the 0° optical activity you're given?
do any of those have the mp of 165 °C
which isomer has solubility = 1.25 kg/L?

if you're still stuck what are OH groups called?  how would you name a dicarboxylic acid molecule with 4 C's and 2 OH groups? get to googling! The answer is in a second wikipedia link which should be relatively easy to find given what I just told you.


« Last Edit: March 21, 2020, 07:25:32 PM by MNIO »

Offline AWK

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Re: Identify a substance
« Reply #7 on: March 22, 2020, 03:39:24 AM »
Quote
It is a white powder
       <probably doesn't contain S since it's white powder right?>

Wrong hint. There are many different colorless organic compounds containing sulfur such as cystine, dimercaptosuccinic acid and so on.
AWK

Offline MNIO

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Re: Identify a substance
« Reply #8 on: March 23, 2020, 06:31:17 PM »
AWK, you're correct.  consider it a generalization based on meatlovers guess of C4H6S3.

The correct answer is of course meso tartaric acid.
« Last Edit: March 23, 2020, 06:49:34 PM by MNIO »

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