more questions

[bec]

what's ribose? i've only ever come across deoxyribose, do we need to know about ribose/ when/where/what it is?

[Mao]

what's ribose? i've only ever come across deoxyribose, do we need to know about ribose/ when/where/what it is?

Ribose is deoxyribose with an OH group on 2' carbon. [hence the name, deoxy-]
it is the sugar in RNA nucleotides.

[bec]

hmmm
and would we need to know what RNA is except that it decodes DNA in order to make proteins?

[polky]

Ribose is the sugar unit that is in RNA (compared to the deoxyribose sugar in DNA).  RNA is sort of a "copy" of a section of DNA that will travel out of the cell's nucleus to the ribosomes, where polypeptides are assembled from amino acids.

I really really really doubt you need to know anything about RNA. At all.

[shinny]

im assuming we're allowed to just say deoxyribose actually, since the study design only says 'structure and bonding of DNA' and theres no mention of RNA at all...so i guess we're not even meant to know it exists =T so don't worry too much about it despite what trial exams have been asking u on it

[Mao]

hmmm
and would we need to know what RNA is except that it decodes DNA in order to make proteins?

it can do more than that [there's actually two types of RNA to begin with], but that's not to worry [not on the course]

basically:
RNA has ribose, not deoxyribose
RNA is single stranded, not double helix
RNA also has four bases, except its A, U [instead of T], G, C [we dont need to know U as its not on the data booklet ]
that's all.

[polky]

mRNA and tRNA lol

There's also pre-mRNA if I remember correctly.

[Matt The Rat]

it can do more than that [there's actually two types of RNA to begin with], but that's not to worry [not on the course]

In fact, there's more. The main 3 are: mRNA (messenger RNA), tRNA (transfer RNA) and rRNA (ribosomal RNA), though.

[shinny]

Questions from the TSFX exams:

Item 1
A 0.150 L flask contains Neon gas at 20.0 C and at atmospheric pressure. When the flask
is heated to 40.0 C , the pressure of the gas will increase by a factor of
A 0.3
B 0.5
C 2
D 8
(does this one even work? i thought temp had to be in K)

Item 3
5.00 g of AgNO3 and 10.00 g of MgCl2 were dissolved in 200.00 ml of water. Assuming
that complete ionisation occurs, the number of ions in solution is
A 1.11×10^23
B 1.63×10^23
C 1.89×10^23
D 2.27×1023

Item 13
A sample of a mixture of alcohols was analysed using Gas Chromatography. The spectrum
obtained is given below. Which of the following statements is false?
A The components separate according to their affinities for the two different phases.
B The smallest molecular weight alcohol will have an Tr value of 15 minutes.
C The area under each curve represents the amount of alcohol.
D The gas used as the mobile phase in this process is most likely to be helium.

Is it just me or is this a horrible worded question? The actual answer is B because ur given a graph and thats obvious, but the problem i had was that A and D could possibly be false too given that in GC, the components don't exactly desorb into the mobile phase but are just swept by it, and that deducing that the mobile phase is likely to be helium is quite an assumption since it could be N2 or some other gas. anyone have anything that disproves these?

[Mao]

Questions from the TSFX exams:

Item 1
A 0.150 L flask contains Neon gas at 20.0 C and at atmospheric pressure. When the flask
is heated to 40.0 C , the pressure of the gas will increase by a factor of
A 0.3
B 0.5
C 2
D 8
(does this one even work? i thought temp had to be in K)

someone should slap TSFX for this

Item 3
5.00 g of AgNO3 and 10.00 g of MgCl2 were dissolved in 200.00 ml of water. Assuming
that complete ionisation occurs, the number of ions in solution is
A 1.11×10^23
B 1.63×10^23
C 1.89×10^23
D 2.27×1023

[AgCl precipitates out]





Magnesium chloride is in excess:

so we have all of , all of , and the in excess, as silver chloride is solid and not present as ions.



molecules

*yes, its a tricky question, thanks polky for pointing out the precipitate, i completely overlooked it xD*

Item 13
A sample of a mixture of alcohols was analysed using Gas Chromatography. The spectrum
obtained is given below. Which of the following statements is false?
A The components separate according to their affinities for the two different phases.
B The smallest molecular weight alcohol will have an Tr value of 15 minutes.
C The area under each curve represents the amount of alcohol.
D The gas used as the mobile phase in this process is most likely to be helium.

Is it just me or is this a horrible worded question? The actual answer is B because ur given a graph and thats obvious, but the problem i had was that A and D could possibly be false too given that in GC, the components don't exactly desorb into the mobile phase but are just swept by it, and that deducing that the mobile phase is likely to be helium is quite an assumption since it could be N2 or some other gas. anyone have anything that disproves these?

D is not wrong, helium is a very commonly used mobile phase, and the statement does say "likely"
and A is not completely wrong. there is affinity towards both stationary phase and mobile phase, even when the mobile phase is inert there can still be dispersion forces...
and the MCQ arent about what is absolutely right, its about which one is the BEST answer [damn you VCAA].


but, this, in comparison with other TSFX questions, isnt that bad. [there are a lot of worse worded questions by TSFX ]

[polky]

Lol mao wrong

For the 2nd question, look at what is in excess.  What precipitates out?  What is left?

[ben4386]

Quick Question from Neap OH trial Exam

Short answer 1D

you have two compounds Isomer I 1,2 dichloroethane, CH2ClCH2Cl and Isomer II 11 Dichloroethane Cl2CHCH2Cl

draw the structural formula of a fragment that would be found in isomer I but not in Isomer II

logically I understand that CH2Cl could be a fragment in I but not two, but i didn't think that logically in when I did this exam. Would CH2CH2 + be another if both of the Cl's on Isomer I were fragmented?

thanks for any help

ben

[Glockmeister]

And one from me. This ones from the TSSM paper.

When methane reacts with chlorine in the presence of UV light,
A an addition reaction occurs to form CH3Cl
B A substitution reaction occurs to form CH3Cl
C the products are CH3Cl and HCl only
D the products include CH3Cl, CH2Cl2, CHCl3, CCl4 and HCL

The answers D. Why is that the case?

[Mao]

Quick Question from Neap OH trial Exam

Short answer 1D

you have two compounds Isomer I 1,2 dichloroethane, CH2ClCH2Cl and Isomer II 11 Dichloroethane Cl2CHCH2Cl

did you mean for isomer II to be CH(Cl₂)CH₃?

even though CH2CH2+ wouldnt be a a fragment found in isomer II, keep in mind that the only way we distinguish from one peak to another is mass/charge ratio. Isomer II can produce a similar ion: CH3CH+, which will be identical to this one, hence if this was an exam, you'd probably get it wrong.

a fragment in the mass spec sense is nothing more than just a number, it doesnt differentiate between different arragements of the same mass

[at least that's what i think]

[Mao]

When methane reacts with chlorine in the presence of UV light,
A an addition reaction occurs to form CH3Cl <--- not addition reaction
B A substitution reaction occurs to form CH3Cl
C the products are CH3Cl and HCl only <---- there are other products
D the products include CH3Cl, CH2Cl2, CHCl3, CCl4 and HCL

of the two choices, option D is the best one, becase there may be cases where B is not necessarily true: substitution reaction of CH3Cl + Cl2 -> CH2Cl2 + HCl for example, which didnt get mentioned.