October 21, 2021, 07:27:37 AM
Forum Rules: Read This Before Posting


Topic: Some IB HL Chemistry Questions (involv. modern analytical, organics, energetics)  (Read 7466 times)

0 Members and 1 Guest are viewing this topic.

Offline Joe22c

  • Very New Member
  • *
  • Posts: 2
  • Mole Snacks: +0/-0
Well I'm going to ask questions from multiple areas so I figured I'd stab them here.

Question 1.
Propanal CH3CH2CHO (M = ?58) undergoes complete fragmentation in a mass spectrometer. What is the m/z value of the most intense line in its mass spectrum?

A. 15
B. 29
C. 43
D. 58

The answer is B. I put C. My reasoning was that the Ch2CHO fragment would account for the mass of 43. Apparently I was wrong.

Question 2

Which reaction has the most negative ?H  value?

A. LiF(s)       ?   Li??(g) ??F? (g)
B. Li??(g) ??F? (g)    ?     LiF(s)
C. NaCl(s)      ?   Na??(g) ?Cl? (g)
D. Na??(g) ??Cl? (g)  ?      NaCl(s)

The Answer is B. I narrowed it down to B or D. How does one determine which? Both involve the formation of bonds and thus are the only possibilities. While Fluorine has greater tendency to react than chlorine, Sodium has a greater tendency to react than lithium. Thus I was uncertain which to choose.

Question 3
A buffer solution can be prepared by adding which of the following to 50 mL of 0.10 M CH3COOH(aq)?

I.   50cm of 0.10mol dm? CH3COONa(aq)
II.   25cm3 of 0.10mol dm?3 NaOH(aq)
III.   50cm3 of 0.10mol dm?3 NaOH(aq)

A. I only
B. I and II only
C. II and III only
D. I, II and III

Answer is B. I thought it was A. How does adding NaOH help the solution to act as a buffer?

Question 4.

Why is H2NCH2COOH NOT a Chiral molecule?

One of the carbons has 4 entirely different neighbouring environments (A hydroxyl group, a carbonyl group, and an Nh2Ch2 group.

Thanks in advance for your *delete me*

Offline Yggdrasil

  • Retired Staff
  • Sr. Member
  • *
  • Posts: 3213
  • Mole Snacks: +483/-21
  • Gender: Male
  • Physical Biochemist
Well I'm going to ask questions from multiple areas so I figured I'd stab them here.

Question 1.
Propanal CH3CH2CHO (M = ?58) undergoes complete fragmentation in a mass spectrometer. What is the m/z value of the most intense line in its mass spectrum?

A. 15
B. 29
C. 43
D. 58

The answer is B. I put C. My reasoning was that the Ch2CHO fragment would account for the mass of 43. Apparently I was wrong.

In a mass spectrometer, a very common fragmentation is alpha cleavage, cleavage of one of the C-C bonds connecting an alpha carbon to the carbonyl carbon.  In propanal, this will result in the formation of a CH3CH2 fragment.

Quote
Question 2

Which reaction has the most negative ?H  value?

A. LiF(s)       ?   Li??(g) ??F? (g)
B. Li??(g) ??F? (g)    ?     LiF(s)
C. NaCl(s)      ?   Na??(g) ?Cl? (g)
D. Na??(g) ??Cl? (g)  ?      NaCl(s)

The Answer is B. I narrowed it down to B or D. How does one determine which? Both involve the formation of bonds and thus are the only possibilities. While Fluorine has greater tendency to react than chlorine, Sodium has a greater tendency to react than lithium. Thus I was uncertain which to choose.

Is the reaction:
Li+(g) + F-(g) --> LiF(s)
or
Li(g) + F(g) --> LiF(s)

Quote
Question 3
A buffer solution can be prepared by adding which of the following to 50 mL of 0.10 M CH3COOH(aq)?

I.   50cm of 0.10mol dm? CH3COONa(aq)
II.   25cm3 of 0.10mol dm?3 NaOH(aq)
III.   50cm3 of 0.10mol dm?3 NaOH(aq)

A. I only
B. I and II only
C. II and III only
D. I, II and III

Answer is B. I thought it was A. How does adding NaOH help the solution to act as a buffer?

What would be the product of a reaction between CH3COOH and NaOH?

Quote
Question 4.

Why is H2NCH2COOH NOT a Chiral molecule?

One of the carbons has 4 entirely different neighbouring environments (A hydroxyl group, a carbonyl group, and an Nh2Ch2 group.

Isn't that three groups?  For future reference, a sp2 carbon cannot be stereogenic.

Offline Joe22c

  • Very New Member
  • *
  • Posts: 2
  • Mole Snacks: +0/-0
Hi, first off thanks for taking time to look at my questions!

Quote
In a mass spectrometer, a very common fragmentation is alpha cleavage, cleavage of one of the C-C bonds connecting an alpha carbon to the carbonyl carbon.  In propanal, this will result in the formation of a CH3CH2 fragment.
Ah I didn't know that. So between the CH3Ch2 fragment and the CH2CHO fragment, the former would just occur more often. Is this always the case in terms of Hydrocarbon fragments vs. fragments cont. C, H, and O ?

The second one was Li+(g) + F-(g) --> LiF(s)

Sorry about lack of sub/superscripts

Quote
For future reference, a sp2 carbon cannot be stereogenic.
Ah I see, thank you very much =)

Quote
What would be the product of a reaction between CH3COOH and NaOH?
Ah I see that that would indeed lead to the formatnion of sodium ethanoate. What's the line that determines when we're adding too much NaOH though? (I understand that III doesn't work because you'd have too many OH ions in sol'n but when does it cross the line? I suppose we need the ka value of ethanoic acid then?)





Offline Yggdrasil

  • Retired Staff
  • Sr. Member
  • *
  • Posts: 3213
  • Mole Snacks: +483/-21
  • Gender: Male
  • Physical Biochemist
Hi, first off thanks for taking time to look at my questions!

No problem.

Quote
Ah I didn't know that. So between the CH3Ch2 fragment and the CH2CHO fragment, the former would just occur more often. Is this always the case in terms of Hydrocarbon fragments vs. fragments cont. C, H, and O ?

In general, you will see alpha cleavage more often than many other types of fragmentation (such as loss of a methyl).

Quote
The second one was Li+(g) + F-(g) --> LiF(s)

If the reaction were Li(g) + F(g) --> LiF(s), then I would agree with your answer.  However, in this case you aren't looking at the ability of the metal to lose electrons and the ability of the non-metal to gain electrons.  Since we're dealing with Li+ and F- the metal has already lost an electron and the F- has already gained an electron.

Here, what we are comparing is the stability gained from bringing a positive charge and a negative charge together.  Now, the stability gained from bringing cations and anions together (the Coulombic potential) is related to two things: 1) the charge of the anions and cations, and 2) the distance between the cations and anions. 

When comparing NaCl to LiF, the charges are the same since both contain a +1 cation and a -1 anion.  Therefore, the difference must be due to the distance between the anion and cation.  Given this information, can you explain why reaction (b) is more exothermic than reaction (d)?

Quote

Ah I see that that would indeed lead to the formatnion of sodium ethanoate. What's the line that determines when we're adding too much NaOH though? (I understand that III doesn't work because you'd have too many OH ions in sol'n but when does it cross the line? I suppose we need the ka value of ethanoic acid then?)

It depends on the concentration of the acetic acid (ethanoic acid) in solution (i.e. the buffer capacity).  Here's an exercise for you: calculate the amounts of ethanoic acid and the amounts of sodium ethanoate in solution when you add 25mL of the NaOH solution versus when you add 50mL of the NaOH solution.

Here would be another calculation that you could try.  In order for a buffer to be effective, the ratio of conjugate acid to conjugate base should be between 1:10 and 10:1 (approximately).  What are the minimum and maximum volumes of NaOH that you could add to your solution from question 3 and still have a effective buffer?

Offline toxic_ninja

  • New Member
  • **
  • Posts: 6
  • Mole Snacks: +0/-0
Ohh that right. You 4th graders had paper 1 &2  on Thursday, and Paper 3 on Friday...

Sponsored Links