Chemistry Examination Discussion

[thushan]

Mmm, Dropbox is down.

Also, for Question 5 - the one about CH3COOH and NaOH, does anyone remember the volume of CH3COOH? The copy I have here has the volume cut off.

[Jeggz]

Mmm, Dropbox is down.

Also, for Question 5 - the one about CH3COOH and NaOH, does anyone remember the volume of CH3COOH? The copy I have here has the volume cut off.

20 ml

[d34thb34st]

upload to another site?

[xenial]

Two of my friends (who are much smarter than me, mind you!) said the purity was actually 87% - because you take the mass of copper deposited and put it over the mass lost from the impure sample. Because if you put it over the total mass you're making an assumption that the reaction came to completion - and perhaps it didn't.

Just some food for thought, I thought it was a lot more sound than what I did! (I got 80.1) Just interested to see what other people think.

[Sentar]

Two of my friends (who are much smarter than me, mind you) said the purity was actually 87% - because you take the mass of copper deposited and put it over the mass lost from the impure sample. Because if you put it over the total mass you're making an assumption that the reaction came to completion - and perhaps it didn't.

Just some food for thought, I thought it was a lot more sound than what I did! (I got 80.1) Just interested to see what other people think.

Yeh, that's what I did, because if you use the total mass you're ignoring the mass that is still on the electrode/is the electrode, which was part of the original measurements as well.

[achre]

Two of my friends (who are much smarter than me, mind you!) said the purity was actually 87% - because you take the mass of copper deposited and put it over the mass lost from the impure sample. Because if you put it over the total mass you're making an assumption that the reaction came to completion - and perhaps it didn't.

Just some food for thought, I thought it was a lot more sound than what I did! (I got 80.1) Just interested to see what other people think.

I'd be really happy if that was the solution, because that's something I don't think I'd have been able to do in the exam.

[thepikanation]

Since apparently the Dropbox isn't working, here's a mirror from good ol' MEGA.co.nz

https://mega.co.nz/#!kRZzCJ5C!FiiNwz7jwVXLj6GuUhM8g2E0HtoFKUeL7IQm1k31rgM

Thanks for the .pdf awesome person from before!

[joey7]

Two of my friends (who are much smarter than me, mind you!) said the purity was actually 87% - because you take the mass of copper deposited and put it over the mass lost from the impure sample. Because if you put it over the total mass you're making an assumption that the reaction came to completion - and perhaps it didn't.

Just some food for thought, I thought it was a lot more sound than what I did! (I got 80.1) Just interested to see what other people think.

If you're assuming that the reaction didn't go to completion, then there could be copper left on the impure sample?

[a-rod]

If you're assuming that the reaction didn't go to completion, then there could be copper left on the impure sample?

There is copper left in the impure sample. That's why the fraction is (mass change of pure copper)/(mass change of impure sample).
The Copper that plated the pure copper electrode is not all of the copper that was/is present in the impure sample, only a fraction of it.

[xenial]

Comes down to how you interpret the question, really! It could be that all the copper came out during the electrolysis, and anything else that came off just fell off in the process. Then it would be 80.1. Or it could be that the mixture of copper/impurities came off at a constant rate, and thus the sample of impure copper at the end had the same proportion of copper in it as the sample at the start did. Then it would be 87.

We'll see in a few days time I guess

[saedf]

Yeah I got something closer to 87% than 81%

[joey7]

There is copper left in the impure sample. That's why the fraction is (mass change of pure copper)/(mass change of impure sample).
The Copper that plated the pure copper electrode is not all of the copper that was/is present in the impure sample, only a fraction of it.

Wouldn't that give the percentage of copper in the substances oxidised into the solution? isn't that neglecting substances from the impure sample that cannot be oxidised?

[thushan]

It's 80.1% for the copper purity sample.

I'm writing up solutions here - this is a tricky question, but my reasoning is here:

"This question is actually fairly tricky. What I gather is that the mass of copper in the impure electrode is more accurately reflected by the amount of copper that has been deposited on the cathode. The reason for this is because the copper deposited on the cathode is a result of the amount of electrons pulled away at the anode by the current. Metals that are more reactive than copper would all dissolve in the H2SO4, and that would account for some of the mass loss in the impure sample. Metals that are less reactive than copper will be, as stated in the following part, not oxidised and will form a sludge."

If you check the electrochemical series, copper is the most reactive element that does not react with H+.

@a-rod: If there is copper left in the impure sample after electrolysis, then determining the percentage purity of copper in the original impure sample would be inaccurate.

[charmanderp]

Ok done (sort of)! Will tidy up my solutions and post them sooooooooooooooooon.

EDIT: OK WAIT NO I MISSED LIKE TWO QUESTIONS.

[xenial]

It's 80.0% for the copper purity sample.

Oh cool, you've made my night! (but our crappy calculators get 80.1 as an answer lol)