Unit 3 Questions MEGATHREAD :)

[Greatness]

Hmmm there's a question like this on VCAA 09 6)a)ii)
'The net charge on a zwitterion is zero, so on aspartic acid it is only the alpha-COOH group which is deprotonated to COO-'
But i've been told by my teacher and read in exams and stuff that the side chains are affected in low/high Ph...

[Water]

In basic/acidic conditions. They are sourced elsewhere, yes. But, it wouldn't be a zwitterion anymore as it would have a single charge? If in a basic or acidic solution for a certain amino acid.

[Andiio]

Since zwitterions are said to have a neutral charge, and if Z/side chain groups are affected, what would you draw if you had to draw say, the aspartic acid zwitterion in a pH of 7?

Obviously the normal NH2 and COOH groups would become protonated/de-protonated, but if you deprotonated the COOH in the Z-group, wouldn't the aspartic acid 'zwitterion' retain a negative charge? :\

[Water]

@Andiio I"m pretty sure, either one of them are affected.



New Questions: From Jacaranda 2011.

The Victorian EPA has recently monitored an oil slick by sample. BY this means the agency was able to idenfy the ship that had ilegally discharged its bilge water into the bay. Which of the following combinations of instrumental analytical techniques would provide the most useful information to identify the hydrocarbons in the oil slick

a) IR and NMR

b) GC And Mass Spectroscopy

c) IR and AAS

d) UV-Vis and HPLC


My question is, why did Jacaranda said B?  I said D.

Isn't Oil a liquid?




2nd Question.

A Chemical analyses a 5.0g ore sample of chalcopyrite, CuFeS2. A gravimetric analysis is carried out so that all the sulfur in CuFeS2 is precipitated as BaSO4. The amount of BaSO4 is determined to be 0.10mol. What is Percentage mass of sulfur in the ore?

a) 3.2%
b) 12.8%
c) 32%
d) 64%


Jacarandaa said 64% D , but I said 32%. Any working out to justify Jacaranda's answer.


I"m assuming its like CuFeS2 --> 2BaSO4

[Mao]

Since zwitterions are said to have a neutral charge, and if Z/side chain groups are affected, what would you draw if you had to draw say, the aspartic acid zwitterion in a pH of 7?

Obviously the normal NH2 and COOH groups would become protonated/de-protonated, but if you deprotonated the COOH in the Z-group, wouldn't the aspartic acid 'zwitterion' retain a negative charge? :\

That depends on the isoelectric point, which you are not required to know.

However, they could word it like "draw aspartic acid at pH=12", in which case the environment is too basic, and all groups deprotonate (net -2 charge).

[In reality, the side group of aspartic acid deprotonates at pH>2.77]

[vea]

Oil is volatile so it can be easily vaporised for GC.

[Mao]

New Questions: From Jacaranda 2011.

The Victorian EPA has recently monitored an oil slick by sample. BY this means the agency was able to idenfy the ship that had ilegally discharged its bilge water into the bay. Which of the following combinations of instrumental analytical techniques would provide the most useful information to identify the hydrocarbons in the oil slick

a) IR and NMR

b) GC And Mass Spectroscopy

c) IR and AAS

d) UV-Vis and HPLC


My question is, why did Jacaranda said B?  I said D.

Isn't Oil a liquid?

Yes, oil in this case is a liquid. However hydrocarbons don't respond very well to UV-vis radiation unless they are conjugated (google carotene to see what this means). IR is mostly useless for hydrocarbons, since it gives functional group information but not too much on overall structure, so you will just see a whole bunch of C-C and C-H peaks. And NMR would only be useful if the mixture can be separated into pure fractions to analyse separately (NMR of mixtures can be ridiculously messy, as the peaks will overlap).

This question is testing a small section of the chromatography chapter, which briefly mentions GC can be paired with MS. With GC-MS, you can achieve a separation of components, as well as the mass spectrum of each components. The separation of components is important because a mixture of different hydrocarbons are present.

[luken93]

Since zwitterions are said to have a neutral charge, and if Z/side chain groups are affected, what would you draw if you had to draw say, the aspartic acid zwitterion in a pH of 7?

Obviously the normal NH2 and COOH groups would become protonated/de-protonated, but if you deprotonated the COOH in the Z-group, wouldn't the aspartic acid 'zwitterion' retain a negative charge? :\

That depends on the isoelectric point, which you are not required to know.

However, they could word it like "draw aspartic acid at pH=12", in which case the environment is too basic, and all groups deprotonate (net -2 charge).

[In reality, the side group of aspartic acid deprotonates at pH>2.77]

So wait, in acidic/basic conditions the Z group is affected, yet as a zwitterion it isn't?

aah chemistry is just getting worse by the minute

[Andiio]

Just some weird question from CSE:

Q: Asparagine is an amino acid that can be produced in the human body. Draw the structural formula of asparagine at pH 2.

I drew it with a protonated NH3+ group in the Z chain and also a 'normal' NH3+ in the a-amino acid... but the answers only drew it with the 'normal' NH3+ (i.e. NH3+CHCOOH)

:\ Am I right or..?

[nacho]

Since zwitterions are said to have a neutral charge, and if Z/side chain groups are affected, what would you draw if you had to draw say, the aspartic acid zwitterion in a pH of 7?

Obviously the normal NH2 and COOH groups would become protonated/de-protonated, but if you deprotonated the COOH in the Z-group, wouldn't the aspartic acid 'zwitterion' retain a negative charge? :\

WAIT, maybe, the NH2 goes to NH3+ from the carboxyl group in the main chain and then to NH42+ from the carboxyl group in the z-group?
Or is my line of thought retarded?

[taqi]

@Water
i got 64%; since u know that its 0.1mol of BaSO4, you know theres a total of 0.1mol of S, so times that by Mr, 0.1x32.1=3.21g, so 3.21/5x100=64%
just what i think

[Water]

Umm yeah, but


but for every 1 amount of sulfur in BaSO4 , and there is CuFeS2 , there shud be two amounts of sulfur in CuFeS2 to get that 1 amount?


Or Is my thinking wrong?

[luken93]

Just some weird question from CSE:

Q: Asparagine is an amino acid that can be produced in the human body. Draw the structural formula of asparagine at pH 2.

I drew it with a protonated NH3+ group in the Z chain and also a 'normal' NH3+ in the a-amino acid... but the answers only drew it with the 'normal' NH3+ (i.e. NH3+CHCOOH)

:\ Am I right or..?

That exam is wrong, it should be protonated on both. I know this for a fact that the exam is wrong.

[Andiio]

Just some weird question from CSE:

Q: Asparagine is an amino acid that can be produced in the human body. Draw the structural formula of asparagine at pH 2.

I drew it with a protonated NH3+ group in the Z chain and also a 'normal' NH3+ in the a-amino acid... but the answers only drew it with the 'normal' NH3+ (i.e. NH3+CHCOOH)

:\ Am I right or..?

That exam is wrong, it should be protonated on both. I know this for a fact that the exam is wrong.

:[

[dmanz]

what is the difference between a strong and concentrated acid and base.

i know that if something is concentrated it means that the soluble particles are closer together.