Please Help-Chem Questions.

[Mao]

i have, but its most probably the professors writing the textbook getting carried away.

something to do with subshells and other things (which also links to donuts somehow, but i forget)

[bec]

The amount of ethanol in a white wine can be determined by colorimetry. A 0.50 mL sample of wine is diluted to 90mL. 10.0mL of 0.100M acidified potassium dichromate is added and the resulting test solution is warmed at 50C for 15min.

a) Why is the test solution held at 50C for 15min before proceeding with the analysis?


(This is question 2 on a past exam but I had a quick look and it's not on the 2002-2007 exams, so the assessment report isn't on the net. could be the 2001 exam 1?)

[cara.mel]

It would be something to do with ensuring the reaction reaches completion

[bec]

thanks caramel

so with the same question, it says that the concentration of dichromate in the test solution is 0.0028M.
Then,
"Calculate the amount, in mole, of ethanol that reacted with dichromate in the original test solution."

How do I do that? I know i need to subtract n(C2H5OH reacted) from n(total C2H5OH) but i don't know how to get the no. mols reacted.

i thought of using n=cV, but i only "c" so that would be impossible.
UNLESS... i assume that there is still 100mL of solution there, even though it's been heated for so long and there would have been evaporation.

is that what i need to do? if not, how do you work it out?

[Mao]

I think you would assume that...

if not, you should check the assessment report...

[bec]

alright, so still doing spectrometry....

I'm trying to write a list of two possible sources of error for all the instrumental spec techniques, can anyone help me fill it up?

AAS: incorrecty positioned line of best fit to creat callibration curve; standard solutions made up incorrectly

UV-VIS: same poss. errors as AAS

Colorimetry: Fingerprints on cuvette, standard solutions made up incorrectly

IR: Incorrect interpretation of graph +

NMR:

thanks

[bec]

1. Which of the following tasks could be completed using UV-vis?
a) Detecting the presence of a performance enhancing drug in a urine sample
b) Determining the concentration of a sodium chloride solution
The answer is (a) - and in the worked solutions the reason given for (b) not being correct is that "AAS can be used to determine the concentration of sodium in a solution".

But does that mean that you COULDN'T use UV-vis to analyse that?


2. Which of the following correctly matches a technique with the required analysis?
a) mass spec to suggest the presence of a CH3 group in an organic molecule
b) UV-vis spec to determine the magnesium content of a fertiliser

Answer is (a), and it says that "magnesium content would be determined using AAS" which i KNOW would be better but does that mean that a UV-vis spectrometer couldn't perform the analysis of magnesium content?

3. Name the spectroscopic technique that would be used to perform the following analyses:
a) Identification of a particular drug in a blood sample (I said AAS, correct A is UV-vis)
b) Detection of the C=O group present in an organic compound (I said NMR, correct is IR)
c) Measurement of the concentration of Mn2+ ions in a waste water sample (I said UV but should be AAS)

Can anyone help clear these up? thanks

[Mao]

from what my understanding, UV spectrum is absorbed by bonds.

metallic ions (and ions in general) a full outershell, making it very stable and have little absorbing potential. metals in AAS however goes through the burner and are reduced to ground state, hence why AAS is very useful for detecting metallic ions.

I'm presuming that UV spectrum are not absorbed by ions. however, transition metals such as copper form colourful complexes, and the bondings in these complexes absorb UV, so *some* metallic ions can be analysed by UV

for q3:
a) since drugs are not metallic ions, AAS would not be used. but as for identification, i thought IR would be more suitable, as it can be used for identification, UV-vis tells the concentration, not molecular structure
b) C13 NMR could be used, but IR is easier, more cost effective
c) metallic ion = AAS. if it was permanganate (or other compounds), UV-vis would be suitable.

[bucket]

Hey guys just practicing some questions for a test on AoS 1.
I seem to have forgotten everything to do with gasses...

i don't know how to do
"calculate the concentration of SO₂ in the polluted air in g m-1"
I have worked out;
m(SO₂)=0.185g
and
v(air)=10.0m³

If you need any other numbers let me know... because i don't know what measurements are required to do such a question.

[Collin Li]

The question doesn't make sense. Concentration is always some amount (i.e.: moles or mass) divided by a volume.

It cannot be . It must be .

A concentration of in is simply:

[bec]

A very basic question but i can't find it in my textbook: what do brackets mean in chemical formulas?

For example, 2-methylhexane is written as CH₃CH(CH₃)CH₂CH₂CH₂CH₃

i was assuming the functional groups were in brackets, but then i saw that 4-nonanol is written as CH₃CH₂CH₂CHOHCH₂CH₂CH₂CH₂CH₃

is this just one of those things that you can write either way, or is there a reason for it?

[Mao]

they usually indicate side-groups

CH₃CH(CH₃)CH₂CH₂CH₂CH₃
the CH₃ in brackets is the methyl side chain attached to the second side-chain, hence 2-methyl hexane

if that wasnt written in that way, it'll be very difficult to figure out what it is, as it can be read as heptene (even though it has two extra hydrogens

as for the 4-nonanol, the OH doesnt need to be in brackets, as it is implied that it is attached to the carbon from the side, not actually being part of the backbone chain itself.

it that made sense at all =)

[bec]

ok cool, thanks