Unit 3 Questions MEGATHREAD :)

[taqi]

exactly what makes DNA acidic or a nuleic acid? i read somewhere its the OH groups on the phosphate group able to donate protons, but i thought hydroxy groups dont donate protons!

[jane1234]

okay kinda confused... how come the two CH2 in butane has a quartet peak but the CH2 in propane has a peak split into 7?

You have to look at which environments are identical.

The CH2 in butane is next to a CH3 (which gives it the quartet) and another CH2.
However, that CH2 is IDENTICAL to the other CH2, as they only account for one peak on the low res 1H NMR. They are in the same chemical environment as each other.
Identical groups won't cause splitting, only the ones that are adjacent and different.

With symmetrical molecules (like butane) I like to cut it in half and count the high res splits that way. So you would just look at the CH3CH2 when deciding splitting, as butane is the same on both sides.

With propane, however, the CH2 is next to two groups that are different from itself, so you take both CH3 groups into account when deducing the splitting pattern.

Hope that made some sort of sense, not sure of the exact theory behind the identical-environment-no-splitting, but that's basically how you work it out...

[lilaznkev1n]

just a random question on what exam to do next:
I'm planning to do vcaa 08 sample on tuesday but not sure which one to do tomorrow.
I've done everything except vcaa 08 sample, tssm 09, tssm 10, tsfx 10.
Ill probably only do two more before the exam so I'll do vcaa 08 (sample) and which other one?

[Greatness]

tssm are terrible imo the questions are written shockingly. stay away from them hahah

[mickeymouse]

okay kinda confused... how come the two CH2 in butane has a quartet peak but the CH2 in propane has a peak split into 7?

You have to look at which environments are identical.

The CH2 in butane is next to a CH3 (which gives it the quartet) and another CH2.
However, that CH2 is IDENTICAL to the other CH2, as they only account for one peak on the low res 1H NMR. They are in the same chemical environment as each other.
Identical groups won't cause splitting, only the ones that are adjacent and different.

With symmetrical molecules (like butane) I like to cut it in half and count the high res splits that way. So you would just look at the CH3CH2 when deciding splitting, as butane is the same on both sides.

With propane, however, the CH2 is next to two groups that are different from itself, so you take both CH3 groups into account when deducing the splitting pattern.

Hope that made some sort of sense, not sure of the exact theory behind the identical-environment-no-splitting, but that's basically how you work it out...

that makes heaps of sense thanks so much!

[thushan]

exactly what makes DNA acidic or a nuleic acid? i read somewhere its the OH groups on the phosphate group able to donate protons, but i thought hydroxy groups dont donate protons!

OH does not donate a proton if it's bonded to carbon, you're right. But this OH is bonded to phosphorus, and it will happily donate a proton (the conjugate base is stable). Same thing for OH bonded to sulfur - i.e. sulfuric acid.

[ttn]

How do you do the pH questions where it asks how many moles of HCl needs to be added to change the pH of a 1.0 L solution from 12 to 2?

[Sakigami]

tssm are terrible imo the questions are written shockingly. stay away from them hahah

QFT. I decided to do a couple of the TSSM exams today, and some of their answers are very wishy-washy.

[luken93]

How do you do the pH questions where it asks how many moles of HCl needs to be added to change the pH of a 1.0 L solution from 12 to 2?

I do c1V1 = c2V2, dunno if there's a better way?

Remember that pH = 1x10^-pH, hence a pH of 2 = 1x10^-2 of H+ ions.

[DavidSheena]

For anyone who has done the KBT 2010; I marked myself harshly but I wanted to know if my answers were sufficient.

d. Indentify two advantages of HPLC over TLC specific to the purpose of identifying the possible danger presented by liquid X.
 My Ans: Hplc provides much more accurate seperation of components which allows for better, specific identification when compared with a standard chromatagram.
Would that be the same as better resolution than TLC so components are easily differentiated?

And when drawing nucleotides: Is it acceptable to leave the Hydrogens on as part of the hydroxyl group? Or should it be O-

[scocliffe09]

For anyone who has done the KBT 2010; I marked myself harshly but I wanted to know if my answers were sufficient.

d. Indentify two advantages of HPLC over TLC specific to the purpose of identifying the possible danger presented by liquid X.
 My Ans: Hplc provides much more accurate seperation of components which allows for better, specific identification when compared with a standard chromatagram.
Would that be the same as better resolution than TLC so components are easily differentiated?

And when drawing nucleotides: Is it acceptable to leave the Hydrogens on as part of the hydroxyl group? Or should it be O-

hydroxyl retains the OH - not an acidic functional group generally.
On the phosphate, however, they are acidic. I think Thushan answered this a few hours ago. Without having done the KBT exam your answer looks decent; resolution and differentiation are far better words to use though.

[ttn]

How do you do the pH questions where it asks how many moles of HCl needs to be added to change the pH of a 1.0 L solution from 12 to 2?

I do c1V1 = c2V2, dunno if there's a better way?

Remember that pH = 1x10^-pH, hence a pH of 2 = 1x10^-2 of H+ ions.

lol I don't get it?

[mickeymouse]

exactly what makes DNA acidic or a nuleic acid? i read somewhere its the OH groups on the phosphate group able to donate protons, but i thought hydroxy groups dont donate protons!

yup I just encountered this in the neap 2008 if it helps the solutions says the OH on the phosphate acts as an acid as it donates a proton, I think you have to take into account the environment of the phosphate.

okay question - the solutions also have that 5.5mg/mL = 0.099mol/L? I got 307.5M

edit: realized I multiplied by molar mass instead of divided sorry

[Lols123]

how did you guys go on VCAA 2009 exam?
just did it (my first timed exam)  and only got 88%
not looking forward to wednesday :/

[extcar]

can someone help me with these questions, answers are there but i don't know how to work it out:
1. Carbon monoxide reacts with oxygen as described by the chemical equation:
2CO(g) +O2 (g) ---> 2CO2 (g)
When 20mL of CO reacts with 80mL of O2 at the same temperature and pressure, what is the volume of the gas mixture after the reaction has reached completion?
Answer is 90mL

2. 5.00g of a mixture of magnesium and zinc was allowed to completely react with dilute sulfuric acid as described by the chemical equation, where M represents either magnesium or zinc
M (s) +H2SO4 (aq) ---> MSO4 (aq) +H2 (g)
If 0.250g of hyrdrogen gas was produced in the reaction, what would the percentage composition of magnesium in the mixture be?
Answer is 37.6%

thanks!

sorry i only read the OT now!  but if anyone wants to give it a try anyway ?