Anyone got their MC solutions?

[charmanderp]

I thought being soluble in water isn't the main reason why it is a good salt bridge, the more important thing is that K+ is a weak reductant so it does not react in preference of say iron or something.

But that option also says that nitrate is a strong oxidant, which would make it a poor choice for an electrolyte.

28 is C, as all the other options are incorrect. D is incorrect because i) oxidation occurs at the anode. Oxidation is a loss of electrons, which means a positive charge builds up at the anode. Thus, anions, which are negatively charged, not K+ (which is a cation) like option D suggests, must flow to the anode to balance this positive charge ii) reduction occurs at the cathode. Reduction is a gain of electrons, which means a negative charge builds up at the cathode. Thus, cations, which are positively charged, not NO3- (which is a cation) like option D suggests, must flow to the cathode to balance this negative charge.

[charmanderp]

For MC I've got:

1. A
2. B
3. A
4. D
5. C
6. A
7. A
8. C
9. C
10. B
11. C
12. A
13. B
14. D
15. D
16. C
17. B
18. D
19. C
20. D
21. A
22. C
23. C
24. A
25. D
26. D
27. B
28. C
29. B
30. D

Hopefully I've transcribed that right from my scrap paper! Short answer coming soon as soon as I finish typing them up (omg so tedious).

[maggieqianyu]

For question 4, still really confused...

II said being 'insoluble in water but soluble in dilute acid', then shouldn't it be something like a gravimetric analysis because something would be precipitated out etc., instead of being a volumetric one?

[charmanderp]

For question 4, still really confused...

II said being 'insoluble in water but soluble in dilute acid', then shouldn't it be something like a gravimetric analysis because something would be precipitated out etc., instead of being a volumetric one?

This question is saying that the sample itself is insoluble in water. So I need to dissolve it using an acidic or basic solvent, and then figure out how much of the solvent was required.

With gravimetric analyses, the sample itself is soluble in water, e.g. fertiliser. So I'd dissolve my fertiliser and then maybe add barium chloride. Barium ions would react with the sulfate ions in fertiliser to form barium sulfate, which is insoluble in water. Then I'd be able to measure how much sulfate was in the fertiliser. But the sample itself when it contained sulfate was soluble in water!

On a simple level, you can't precipitate a sample that isn't soluble in water.

So if I had a limestone sample and I dunked it in water, it would just be a big ol' rock in water. Hence what I'd need to do is dissolve it in acid instead. The reason it dissolves in acid is because acids are more corrosive than water, and more importantly calcium carbonate is a base. So I'd add a known excess of acid to dissolve the limestone, then titrate that excess with a base to figure out how much acid was left over after dissolve the sample. I'd then be able to calculate how much was actually required for the sample, thus how much calcium and/or carbonate was in the limestone (save for contaminations).

[thecreeker]

Could you explain why 20 is D please? I resorted to just going with the highest conc of ethanoic acid but that's obviously wrong haha

and what do you reckon is a good score on the multiple choice judging by the standard of the exam for an A+?

[charmanderp]

Could you explain why 20 is D please? I resorted to just going with the highest conc of ethanoic acid but that's obviously wrong haha

and what do you reckon is a good score on the multiple choice judging by the standard of the exam for an A+?

Since ethanoic acid is a weak acid, you could use the Ka values in the data booklet to work out that it's D, figuring out the [H3O+] for each one, then dividing by the initial concentration of acid and multiplying by one hundred.

EDIT: Omg my logic for the second bit is so poorly explained it doesn't sound right at all. Trying to think of a better way to explain. But the answer is definitely D!

[saedf]

Yeah I just resorted to the highest concentration favoring a net forward reaction for Q. 20, although I didn't specifically do calculations with Ka values to confirm

[maggieqianyu]

So if I had a limestone sample and I dunked it in water, it would just be a big ol' rock in water. Hence what I'd need to do is dissolve it in acid instead. The reason it dissolves in acid is because acids are more corrosive than water, and more importantly calcium carbonate is a base. So I'd add a known excess of acid to dissolve the limestone, then titrate that excess with a base to figure out how much acid was left over after dissolve the sample. I'd then be able to calculate how much was actually required for the sample, thus how much calcium and/or carbonate was in the limestone (save for contaminations).

Oh thanks that makes so much sense... got so messed up with all the conceptions 

[Stick]

Could you please explain Question 13 of multiple choice? I must've stuffed up my oxidation numbers because I got that they were all reduction reactions. XD

[simba]

Could you please explain Question 13 of multiple choice? I must've stuffed up my oxidation numbers because I got that they were all reduction reactions. XD

Thank god I'm not the only one who got that haha, I thought I was going crazy

[Stick]

By the sounds of things, I think we got a lot of the same answers. XD

[charmanderp]

Could you please explain Question 13 of multiple choice? I must've stuffed up my oxidation numbers because I got that they were all reduction reactions. XD

Which ON were you using?

[Stick]

In NaNO3, I got an oxidation of +5 for N; in NO2, I got an oxidation of +4; in NH2, I got an oxidation number of -2. I think I made an error here though because in NH the oxidation number is -1... my logic is probably wrong.

[simba]

By the sounds of things, I think we got a lot of the same answers. XD

TWINS!

[decentxion]

Which ON were you using?

I used the N oxidation number for step 1, reducing from an ON of +5 to +4.. then in step 2, again looking at N, reducing from an ON of +4 to +2... then the only MC option that had both step 1 and 2 was D.
I feel like that isn't right... but someone have a check?