Chem questions

[bubbles]

Can someone please explain how to obtain the answers for these following questions:

1. A sample of 1.00g of a pesticide is analysed for its arsenic content by precipitation of the arsenic as the sulfide, As2S3. If 0.123g of sulfide is obtained, the percentage by mass of arsenic in the pesticide is:
A. 3.75
B. 7.50
C. 37.5
D. 75.0

The answer is B
2. If 3 mole of Fe2O3 and 2 mole of CO are mixed together, 1 mole of Fe is produced. Calculate the amount of substance remaining at the end of the reaction.
3. Consider the following reaction: 4NH3 (g) + 5O2(g) --> 4NO2(g) + 6H2O(g)
Assuming that the pressure and temperature remain constant, if 40cm^3 of ammonia is mixed with 70cm^3 of O2 and ignited, the final volume of the gas mixture will be:
A. 120cm^3
B. 60cm^3
C. 210cm^3
D. 100cm^3
The answer is A

If you were asked in the exam to draw the structural formula for the formation of a triglyceride- is it acceptable to leave it in its semi-structural form or would we have to draw the entire R group?

[ed_saifa]

If you were asked in the exam to draw the structural formula for the formation of a triglyceride- is it acceptable to leave it in its semi-structural form or would we have to draw the entire R group?

I think that it is acceptable to draw it in semi structural form providing the question doesn't all for it to show all bonds

[iamdan08]

Q1) 
                                 
                                     
                                                          
 

             
               

%      %                       

[bubbles]

Isn't "0.123g" the mass for 3S^-2 (sulfide) not As2S3?

[Mao]

you'd think so

but the numbers are too nice to be that case.

[bubbles]

In the 08 STAV short answer 8 b)
Give one chemical test that can be used to distinguish between each of the following pairs of compounds. Describe the results that would be expected in each test. Write appropriate give chemical equations for the reactions that are involved.
i) butan-1-ol and butanoic acid
iii) butan-1-ol and methylpropan-2-ol

for i would IR be correct and iii NMR?- would they be considered as chemical tests?

The answers provided by STAV sounds a little foreign to the chemistry course... 

[Mao]

yes, these couple questions arent actually on the chemistry course [but STAV likes these stuff.......!]

i) Butan-1-ol and Butanoic acid:
add Sodium carbonate, the acid will bubble whereas the alkanol will not react:



ii) butan-1-ol and methylpropan-2-ol:
add acidified dichromate solution, butan-1-ol will oxidise to become butanoic acid, whereas methylpropan-2-ol will become a ketone



then add sodium carbonate. the butanoic acid [which used to be Butan-1-ol] will form bubbles [see previous equation], whereas the ketone will not react.

[ben4386]

butan-1-ol and methylpropan-2-ol

if you oxidise butan-1-ol with something like acidified potassium dichromate you should get butanoic acid

If you do the same to methylpropan-2-ol    CH3C(CH3)(OH)CH3, if you tried the same thing with methyl propan-2-ol there would be enough bonds on the middle carbon to form a C=O hence it won't form an acid. So I would imagine that if you had two of them in two beakers but you didnt know which one was which, you could oxidise them, and then test the acidity of them.

[shinny]

for the original question about the triglyceride, i dont think its actually possible to expand it (unless its a saturated fatty acid) because u wouldnt know where the double bonds are, and whether theyre even double bonds or if theyre triples etc. well, that and the fact that the chem exam isnt printed on A2 paper.

[Mao]

for the original question about the triglyceride, i dont think its actually possible to expand it (unless its a saturated fatty acid) because u wouldnt know where the double bonds are, and whether theyre even double bonds or if theyre triples etc. well, that and the fact that the chem exam isnt printed on A2 paper.

I dont think VCE chemistry deal with fatty acids with triple-bonds, and its fairly trivial to determin the number of double-bonds [saturated, mono-un or poly-un]

as for drawing it out, you would leave it in the form the data booklet gives to you [R-COOH]
however, it may tell you the location of the double-bond(s) and ask you to write the semi-structural formula
the likelihood of asking you to draw the full structural formula, it'll take too much time that VCAA know they're better off assessing other key knowledges rather than seeing how well you can draw 17 repeating units of CH2s...