Unit 3 Questions MEGATHREAD :)

[DavidSheena]

I have a few questions
1) Name a functional group present in all four nitrogen bases found in DNA.
The answer was Amine but I thought it would be carbonyl because in the data book I couldn't determine the amine group present on thymine.

2)Gravimetric analysis: why where multiple weighings of the precipitate formed?
ANS: Ensure precipitate was dry.
I wrote to acheive constant mass. Would that be acceptable?

3)How would an appropriate wavelength be determined for AAS analysis?

The answer didn't state anything about cathode lamps. And it was a bit too all over the place to understand.

[Vincezor]

I have a few questions
1) Name a functional group present in all four nitrogen bases found in DNA.
The answer was Amine but I thought it would be carbonyl because in the data book I couldn't determine the amine group present on thymine.
There is no carbonyl functional group in the adenine base? :S. Sorry I'll have to leave this for others to answer.

2)Gravimetric analysis: why where multiple weighings of the precipitate formed?
ANS: Ensure precipitate was dry.
I wrote to acheive constant mass. Would that be acceptable?
I'd say so. constant mass is achieved when the precipitate is fully dried.

3)How would an appropriate wavelength be determined for AAS analysis?
The answer didn't state anything about cathode lamps. And it was a bit too all over the place to understand.
:S Question sounds like something you'd ask in UV-Vis analysis. What exam was this? For AAS you'd use the appropriate hollow cathode lamp for the metal being analysed, so I'm not sure what else you need to say...

[luken93]

1) It is present in thymine in the form -NH-, this is still an amine, however the NH2 is a primary amine, whereas -NH- is a secondary amine (connected to 2 carbons)

2) Yeah, I'd say yours is better than the answer

3) What Vince said.

Overall, it was a shit exam. Take their answers/questions with not too much thought.

[tony3272]

Just to add to this, i'm pretty carbonyl is not considered a functional group.

[nacho]

are we required to know that a nickel catalyst is present in hydrogenation from ethene to ethane?

[Matt-Ong]

TSFX 2009 Exam

Question 3:
Which of the following statements is incorrect?

B. The oxidation number of N in NH3 is -1/3
D. The oxidation number of H is LiAlH4 is +1

The answer says D and that the oxidation number can be a fraction.
I know that the oxidation number can be a fraction, but if N = -1/3, and H = +1, then won't the charge be 8/3?

[DavidSheena]

Thanks for clearing up Quesiton One. My bad it clearly isn't carbonyl, I was thinking of C=C. Some random exam said that C=C was a functional group present in nucleotides.

And for question 3. It was UV-Vis; read it wrong. Excuse my stupidity . But what would the answer be for UV- Vis?

[nacho]

Thanks for clearing up Quesiton One. My bad it clearly isn't carbonyl, I was thinking of C=C. Some random exam said that C=C was a functional group present in nucleotides.

And for question 3. It was UV-Vis; read it wrong. Excuse my stupidity . But what would the answer be for UV- Vis?

you keep picking wave lengths until a maximum abosrbance is determined...  i think

TSFX 2009 Exam

Question 3:
Which of the following statements is incorrect?

B. The oxidation number of N in NH3 is -1/3
D. The oxidation number of H is LiAlH4 is +1

The answer says D and that the oxidation number can be a fraction.
I know that the oxidation number can be a fraction, but if N = -1/3, and H = +1, then won't the charge be 8/3?

there must be a mistake.. It would be -3 for N.
Hydrogen's oxidation number is always +1, except for metal hydrides, if i am not mistaken?

__ _ _ _

go call mao someone..

[thatricksta]

I have a few questions
1) Name a functional group present in all four nitrogen bases found in DNA.
The answer was Amine but I thought it would be carbonyl because in the data book I couldn't determine the amine group present on thymine.

2)Gravimetric analysis: why where multiple weighings of the precipitate formed?
ANS: Ensure precipitate was dry.
I wrote to acheive constant mass. Would that be acceptable?

3)How would an appropriate wavelength be determined for AAS analysis?

The answer didn't state anything about cathode lamps. And it was a bit too all over the place to understand.

I believe I just finished this exam and it mentions of a PURPLE organic compound, its corresponding wavelength will be YELLOW, i do not know the wavelength of this by heart... the exam overall (NEAP 2008 I assume) is not a very good one, its full of junk

I wrote the same answer for question two and thought it was significantly better than theirs!

[Matt-Ong]

there must be a mistake.. It would be -3 for N.
Hydrogen's oxidation number is always +1, except for metal hydrides, if i am not mistaken?

That's what I thought as well.. NH3 is definitely not a metal hydride.. So.. TSFX got it wrong?

[thatricksta]

NEAP 2008 has a question which specifies that C4H604 will have an IR band at 1700 which indicates a C=O bond, it then also states that 0.134g dissolved in water is titrated with a standardised 0.106 M NaOH solution. A titre of 21.43 was required to reach the end point.

B) "Does the information provided about experiment 2 allow you to state the functional group present in the compound?

I wrote:
Yes. The IR spectrum indicates a C=O bond. The compound reacts with NaOH which is a base, indicating that the compound is an acid. Therefore it will have a carboxyl functional group

NEAP answer:
"The band at 1700 is due to the carbonyl group. As this group is present in both esters and carboxylic acids (and other groups such as amides, aldehydes and keytones), we cannot be sure which functional group is present."


Who is correct? Can someone please explain this to me? Do I not understand the significance of the titration in this experiment? THANKS!

[thatricksta]

Also, the final question on this exam states

"How would an appropriate wavelength be determined for this analysis?"
This is in respect to an experiment using "spectroscopy" (not specified) and indicates that the compound being tested is "bore water", containing iron (we are determining the concentration of Fe in the bore water), which is first added to ferrozine to form a PURPLE complex. It goes on to suggest "their absorbances were determined in a spectrometer using an appropriate wavelength of light" (which I would assume corresponds to UV-Visible spec??)

NEAP writes its answer as:
A graph of the absorbance versus wavelength for a solution of iron/ferrozine complex would be obtained (1 mark)
The absorbance of maximum wavelength would be chosen (provided that no other component of bore water sample absorbed this wavelength) (1 mark)

ME:
I straight away recognised the complimentary colour yellow and wrote that
Yellow would be used as it is complimentary to purple.

Is my answer OK? Or is it straight up WRONG?
Thank you

[nacho]

Also, the final question on this exam states

"How would an appropriate wavelength be determined for this analysis?"
This is in respect to an experiment using "spectroscopy" (not specified) and indicates that the compound being tested is "bore water", containing iron (we are determining the concentration of Fe in the bore water), which is first added to ferrozine to form a PURPLE complex. It goes on to suggest "their absorbances were determined in a spectrometer using an appropriate wavelength of light" (which I would assume corresponds to UV-Visible spec??)

NEAP writes its answer as:
A graph of the absorbance versus wavelength for a solution of iron/ferrozine complex would be obtained (1 mark)
The absorbance of maximum wavelength would be chosen (provided that no other component of bore water sample absorbed this wavelength) (1 mark)

ME:
I straight away recognised the complimentary colour yellow and wrote that
Yellow would be used as it is complimentary to purple.

Is my answer OK? Or is it straight up WRONG?
Thank you

to be honest, i would have put what the neap answers suggested (as stated in my previous post)
I wouldnt think your answer would get the mark, UNLESS you added what wavelength of light 'yellow' occurs at. Yellow is a colour, not a wavelength.
And complimentary colours/corresponding wavelengths are not in the study design AFAIK.
hopefully someone can verify what i've said

[Water]

Yeah bitch! One of the Companies discretely added the complementary colour and I was LIKE ASJFGASKJGASJKFAJSGAHGSJKG, your screwing with me right? I have to know complementary colours. Lolol. When I did two VCAA Exams, I realized, I didn't need to .

[luken93]

NEAP 2008 has a question which specifies that C4H604 will have an IR band at 1700 which indicates a C=O bond, it then also states that 0.134g dissolved in water is titrated with a standardised 0.106 M NaOH solution. A titre of 21.43 was required to reach the end point.

B) "Does the information provided about experiment 2 allow you to state the functional group present in the compound?

"In experiment 2, the infrared spectrum of the compound was obtained. The spectrum included a band at a
wavenumber of approximately 1700 cm -1 ."

Experiment 2 is only the IR, you're jumping ahead of yourself.

Also, the final question on this exam states

"How would an appropriate wavelength be determined for this analysis?"
This is in respect to an experiment using "spectroscopy" (not specified) and indicates that the compound being tested is "bore water", containing iron (we are determining the concentration of Fe in the bore water), which is first added to ferrozine to form a PURPLE complex. It goes on to suggest "their absorbances were determined in a spectrometer using an appropriate wavelength of light" (which I would assume corresponds to UV-Visible spec??)

NEAP writes its answer as:
A graph of the absorbance versus wavelength for a solution of iron/ferrozine complex would be obtained (1 mark)
The absorbance of maximum wavelength would be chosen (provided that no other component of bore water sample absorbed this wavelength) (1 mark)

ME:
I straight away recognised the complimentary colour yellow and wrote that
Yellow would be used as it is complimentary to purple.

Is my answer OK? Or is it straight up WRONG?
Thank you

Nah, I wouldn't say you'd get the mark either. The important aspect of choosing a wavelength is that it is a) Strong absorbance, and b) No other compounds absorb strongly at this wavelength. As for complimentary colours, although it may absorb strongly, it's not necessarily true that it would be the best.