Anyone got their MC solutions?

[Mafioso]

for the question which asks when back titration should be used, i wrote ii) and iii) because i dont see how something being volatile makes back titration better than normal titration. anyone else?

100% sure that it is used for volatile substances.

[Scanlia]

So from looking at adam_s95, saedf, Chazef, and randomposter's chosen answers, I read through the exam and I think the answers for the multi-choice are:



Though I'm not 100% sure.. Esp. with Q20 & 25:

[Alwin]

So from looking at adam_s95, saedf, Chazef, and randomposter's chosen answers, I read through the exam and I think the answers for the multi-choice are:
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WHERE DID YOU GET THOSE PICS FROM?
can you please post the full thing?
nvm, found it... can't believe got q2 wrong

Oh, and mine were:

A C A C C A A C C B C A B D C B D C D D C C A B D B D B C B D

lol... I think I messed up a few

[randomposter]

I'm pretty sure #30 was C. Because Fe has a charge of two whereas the others have a charge of one, and so only half the number of moles of Fe will be deposited compared with K and Ag.

[Sentar]

I'm pretty sure #30 was C. Because Fe has a charge of two whereas the others have a charge of one, and so only half the number of moles of Fe will be deposited compared with K and Ag.

Fairly sure it was D, Potassium doesn't form a solid as water reduces in preference to it.

[randomposter]

Fairly sure it was D, Potassium doesn't form a solid as water reduces in preference to it.

ahhh... tricky. 

[Alwin]

I'm pretty sure #30 was C. Because Fe has a charge of two whereas the others have a charge of one, and so only half the number of moles of Fe will be deposited compared with K and Ag.

No potassium formed since it is below water on the electro chem series. And it was asking for mass iirc, so you had to times it by the molar mass too... don't think it made much of a difference tho

[Sentar]

So from looking at adam_s95, saedf, Chazef, and randomposter's chosen answers, I read through the exam and I think the answers for the multi-choice are:

(Image removed from quote.)

Though I'm not 100% sure.. Esp. with Q20 & 25:

(Image removed from quote.)

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Fairly sure 25 is D. It doesn't oxidise water in preference to itself, and they are standard half cells.

Also are you sure 28 is D, I think it's C, but might be wrong. Also not sure about 20, I put D but again could be wrong and not confident.

[bubblecups1]

28 seems like a tricky question, I though it was B because all the salt bridge has to do is to provide a steady stream of anions/cations, hence the main reason why KNO3 is used is because it is unreactive.

[eddybaha]

pretty sure answer for question 20 is d.
1. obviously we can eliminate b and c as they different volumes by same conc. as volume does not affect %ionisation
2. comparing a and d, since the k_a values are the same for both we can set up equilibrium constant expressions
for a) k_a=[H₃O⁺]²/0.01
therefore [H₃O⁺]^{=0.1k_a
% ionisation=0.1k_a/0.01 x 100=1000k_a
for d) k_a=[H₃O+]2/1
therefore [H₃O+]=k_a
% ionisation=k_a/1 x 100=100k_a}

[GGlyfe]

but for q 28 dont the cations of the salt bridge flow to the cathode side? and the anion of the salt bridge flow to the anode? cause since anode is the site of oxidation, anion will need to be released to balance the positive charge formed due to the oxidation. vice versa So i thought it couldnt be D so i chose C as the best answer.

[bubblecups1]

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.

[Sentar]

but for q 28 dont the cations of the salt bridge flow to the cathode side? and the anion of the salt bridge flow to the anode? cause since anode is the site of oxidation, anion will need to be released to balance the positive charge formed due to the oxidation. vice versa So i thought it couldnt be D so i chose C as the best answer.

I chose C but didn't even realise this bit, you're right NO3- flows into the anode half cells, as positive chargers are 'formed' there.

[Cazalinko]

I'm pretty sure 30 is C. I know water is oxidised over potassium normally, but I thought at a high molar concentration such as 1M, potassium was favoured over water? I'm not 100% sure on this though, so I don't know...

[Sentar]

I'm pretty sure 30 is C. I know water is oxidised over potassium normally, but I thought at a high molar concentration such as 1M, potassium was favoured over water? I'm not 100% sure on this though, so I don't know...

That's true for Cl-, where there is a small difference in half cell voltages (0.13V), but there is a big difference between water and potassium (over 2v)