TrueTears question thread

[chem-nerd]

thanks for that /0

just another question

Say you got an amide group -CONH-, but when you put that through a IR spectrometer would there be a peak between 1670 - 1750 which is the C=O peak?

The sample exam for VCAA 2008 says no, which is the answer I got after a process of elimination, since all the other answers were wrong, but I just want to make sure where exactly does the C=O on the data sheet 1670 - 1750 corresponds to? Is it only for carboxy or what?

Thanks

are you talking about MCQ 17? if so, i think you've misinterpreted it. the answer does say that it will have a peak for C=O at about 1700cm^-1

[TrueTears]

Yes it is that question. The question says 'which of the follow is NOT correct' so that means the statement about the C=O peak at 1700cm^{-1} is incorrect. I thought it would be correct?

[chem-nerd]

it is correct. option B is the incorrect one

[TrueTears]

Oh god I need to read more carefully lols.

Thanks chem-nerd for all your help XD



oxidation numbers for every element from left to right is -1 , +1 , -2 , 0 , -2 and +1

For the hydrogens which of one is to which? As in is it the being oxidised to and being reduced to or is it being oxidised to and being reduced to ? How do you tell?

Thanks guys!

[Mao]

H2O donates a H+ to the H-. In this case, H- is oxidized to H2, and the H in H2O is reduced to H2.

The other case involves removing both H+ from the water, the activation energy will be greater as the intermediate is thermally less stable.

[TrueTears]

A 20.0 mL sample of acid is delivered to a lab for analysis. The identity of the acid, and its concentration is not known. The 20.0 mL sample is made up to the mark with water in a 250 mL volumetric flask. 25 mL aliquots are then titrated against 0.05M of sodium carbonate, Na_2CO_3, solution. Methyl red is chosen as the indicator and a sharp transition occurs at an average titre of 15.6 mL. If the acid is:
a) monoprotic, what is the concentration of diluted acid.
b) diprotic, what is the concentration of the diluted acid.

Thanks!

[monokekie]

a) monoprotic acid:

  methyl red indicator suggests that this could be a reaction between a strong acid and a relatively weak base.
  
  HCl could be the acid, since it meets both requirements, a strong acid and a monoprotic acid.
  Then write an equation between HCl and Na2CO3. the mole ratio n(Na2CO3)=(1/2)n(HCl)

  n(Na2CO3)= 0.0156x0.025
  n(HCl)=2x0.0156x0.025
  c(HCl)=(2x0.0156x0.025/0.25)x0.25/0.02 mol L

b) a diprotic acid:

   vice versa, the acid could be H2SO4

^^; i hope this helps lol

[TrueTears]

a) monoprotic acid:

  methyl red indicator suggests that this could be a reaction between a strong acid and a relatively weak base.
 
  HCl could be the acid, since it meets both requirements, a strong acid and a monoprotic acid.
  Then write an equation between HCl and Na2CO3. the mole ratio n(Na2CO3)=(1/2)n(HCl)

  n(Na2CO3)= 0.0156x0.025
  n(HCl)=2x0.0156x0.025
  c(HCl)=(2x0.0156x0.025/0.25)x0.25/0.02 mol L

b) a diprotic acid:

   vice versa, the acid could be H2SO4

^^; i hope this helps lol

ahhh that helped alot! yeah I just wasn't sure how to determine the specific acid, but now I get it. Thanks monokekie!

[TrueTears]

H2O donates a H+ to the H-. In this case, H- is oxidized to H2, and the H in H2O is reduced to H2.

The other case involves removing both H+ from the water, the activation energy will be greater as the intermediate is thermally less stable.

Thanks, but how do you know which is being oxidised/reduced to which? Why can't it be the other pair?

[ryley]

I think you have to look at the oxidation numbers. In water, the oxidation number of H is +1, and in hydrogen gas its zero, so as its oxidation number has been reduced, it has undergone reduction. For the H-, its oxidation number has gone from -1 to zero, its increased so its been oxidised. Thats how I think about, someone else could probably explain it better though.

[Mao]

H2O donates a H+ to the H-. In this case, H- is oxidized to H2, and the H in H2O is reduced to H2.

The other case involves removing both H+ from the water, the activation energy will be greater as the intermediate is thermally less stable.

Thanks, but how do you know which is being oxidised/reduced to which? Why can't it be the other pair?

oxidation is loss of electron. In this case, the H- loses some of its electron density to H+, hence it is oxidized. The H+ gains some electron density from the H-, hence it is reduced.

[TrueTears]

Thanks ryley and Mao!

Another one:

Which compound is a member of the same homologous series as 1-chloropropane?
a. 1-chloropropene
b. 1-chlorobutane
c. 1-bromopropane
d. 1,1-dichloropropane

I said b. but answers is d. I thought that if it is in the same homologous group means that the compound differs by a group.

Thanks!

EDIT: just another Q, does it matter how many sig figs we put to in my working? Say the least accurate was 3 s.f, but if I put like 5 s.f in my working but final answer is 3 s.f would that matter?

[arthurk]

i saw that question on an exam as well don't rememeber where
i saw the answer and was like

Plus i don't think it matters how many sig figs you use in working as long as your final answer contains the necessary sig figs
however im wondering which types of numbers should not be used in determining the necessary number of sig figs
such as absorbance etc always has 2 sig figs but i don't think this value is ever taken into account, even in the 2009 neap trial exam it mentioned 3m^3 somewhere but then the answer was to like 3 sig figs.

[TrueTears]

^ Thanks. So is the answer wrong?

Also:

A particular gene consists of 30 nucleotides. How many molecules of water would be required to completely hydrolysis this gene?

Does this mean single stranded DNA? Or double stranded? Because if it is single you will need 29 to hydrolysis each nucleotide then 2 water molecules to completely seperate the sugar, phosphate and base. But if it is double stranded then you will need 28 water molecules to hydrolysis each nucleotide etc.

Thanks!

[arthurk]

yeah im sure the dichloro question is wrong cause on another exam i just did it told me to define homologous series
all it said was differs by CH2. Your choice would be the only one that corresponds with that definition thus must be the correct answer

This next question is quite ambiguous
which exam?