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AQA Chemistry Practical - Discussion Thread

Alas, the exam is over. What did we all think of it?

I felt that it followed the same format as previous years, and am glad that my process of elimination technique worked no ends, I would have been screwed otherwise.

2002 - Titration, Enthalpy Change (Check this one out as the plan we just completed was virtually the same)

2005 - Titration, Enthalpy Change

2008 - Titration, Enthalpy Change.

Can you see the pattern, rather lucky I guess, as I basically only revised this.

All in all it was a pretty nice paper, although there were some bits that I felt might have tried to catch people out. Usually the % error of the apparatus is less than the overal experiment, and thus we say that there could have been errors in other places, but no the guy performing this experiment clearly was a chemistry God!

My average titre 22.65 (although it goes on what your teacher has, or class average)

Some random figures I remember - from analysis and evaluation:

Averge titre: 22.70 cm^3
Moles: 1.135 x E -3
Mr: 132
Water of Crystalisation 1.444 {or something with .444 in} (obsure) so 1 because we round.
Apparatus % error 6.67 + 0.66 = 7.3%
% by difference = 6.5%

How did we all do - I had this exam this morning and have sat in isolation all day as had General Studies also (failed at that)

KJDM

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Reply 1

thetoothfairy

yep, altho moles to 3sf, so should be 0.00114? And I got 132.2 Mr, 7.4% error, and then 6.6% error....will I get marked down for being 0.1% out?

Reply 2

thetoothfairy

oh ad I forgot to put a minus sign for the last one!

And whats the titration? I got 25.1? How far am I out?

Reply 3

Prof Milne

Your values don't see too far out - for the analysis section, the markschemes are usually fairly manipulative in their workings, as long as your accuracy isn't diabolically terrible.

Indeed, I also got 132.2 but I rounded down to 132 for ease of use with numbers, the markschemes allow this, and usually they are only looking for 1d.p so anything better than this I doubt will affect the markings, as long as it rounds to what the markscheme says.

Your titre average for the implimenting exercise seems relatively high compared to my schools averages, however it would be wiser to compare it to what your friends got in your school.

What volume of CuSO4 did you use in your planning and hence what mass of zinc? Also did you extrapolate your curve,

I forget to mention a few vital points in the method like "stir the solution" however I feel that I may have picked up the marks in other places.

And on the last part where we had to use q= -mcΔT for the constant I used 4.12KJmol-1 rather than 4.18KJmol-1 - which was little bit of a bugger so may have lost a few points on that too.

Overal I will be happy with anything above 25/30 Raw - I really need to revise module three however as this is my weak point.

KJDM

Reply 4

kevster

Prof Milne

do you know what alas means?

Reply 5

Prof Milne

Indeed I do - it means "disspointment" or "unfortunately"- I hope you understand that I said this in a sarcastic manner.

Reply 6

kevster

yea, why the heel are you disappointed the exam is over.

Reply 7

Prof Milne

As previously stated it was said in sarcasm.

How did you find the exam?

Reply 8

username129099

have a guess who didn't use the dilute acid in his titration, ahhh yeah!!! But yeah your values look abou the same as mine(which isn't a good thing). I hate practicals, i just do everything wrong because i panic

Reply 9

useful_idiot

Prof Milne

Water of Crystalisation 1.444 {or something with .444 in} (obsure) so 1 because we round.

Yeh, i got 1.45555...pretty similar.
Anyway, i said it would be a 1:1.5 ratio, so basically:

2CuSO4.3H2O

Don't know whether thats right, but i didnt thing rounding down by 0.46 was pretty sensible.

Everything else i got the same...average titre for implementing 25.20, i think it will have boundaries from about 24.70-25.30 to get the mark.

Reply 10

TheBoot

useful_idiot

no 2CuSO4.3H2O is wrong, it asked for how many moles of h20 present with one mole of copper sulphate, it was correct to round down which is what i did.

Reply 11

merc123

I put 131.6 down for the Mr, is that ok?

Reply 12

TheBoot

merc123

I put 131.6 down for the Mr, is that ok?

thats exactly right me thinks

Reply 13

useful_idiot

TheBoot

no 2CuSO4.3H2O is wrong, it asked for how many moles of h20 present with one mole of copper sulphate, it was correct to round down which is what i did.

yes, i left my answer as 1 mole CuSO4:1.5 moles H2O...but it's like an equation where you double the values to get a whole number, thats how i tried to explain it, like an empirical formula calculation.

As i said, rounding down by nearly half a mole is, in my opinion, just stupid...but if that's the answer, gutted. 29/30 max for me.

and i, and most in my session at school for 132.2 for the Mr.

Reply 14

merc123

To me it seems like AQA are recycling the papers because the 2001 plan cropped up last year and the 2002 plan cropped up this year, what do you lot think?

Reply 15

username129099

TheBoot

thats exactly right me thinks

i suppose it depends on which results you said were concordant i got average titre of 22.70. so that*conc(0.1?)/1000/2 i think..I got 132.2

Reply 16

merc123

I think i got 131.6 for the Mr because i used 1.14 x E-3 mol to calculate the Mr thinking that the answers should always be 3 sig figures.

Reply 17

flyingwhisk

So glad its over and wasnt too bad on the whole- except for a certain easy question where you had to work out the average as i used all the results instead of just concordant :frown:

and i cant beleive i made that mistake (feel like such an idiot).
but my practical itself was good and i got 25.275 as my average. do you know how the marking works with raw/ums?

Reply 18

Hedgeman49

flyingwhisk

So glad its over and wasnt too bad on the whole- except for a certain easy question where you had to work out the average as i used all the results instead of just concordant :frown:

and i cant beleive i made that mistake (feel like such an idiot).
but my practical itself was good and i got 25.275 as my average. do you know how the marking works with raw/ums?

Pobjoy?

I got 25.65 as my average.

Reply 19

Prof Milne

I think we are those who got: 131.6 and 132.2 are both right as we just used different accuracies and they both round to 132 anyway so I don't see any problem there.

Use_Idiot - it seems you have gone a little around the houses to get to the destination. But if you left your answer as 1 mole I feel you should earn some marks, although I do feel the markscheme will not have your method.

Yes - AQA always recycle the practical exercises they are rarely that different from year to year - which I am not complaining about, this year was the last year of that syllabus so I don't think they would have made much effort, and you have to think that it is difficult for some people to get hold of the 2002 paper as it is so old, however I posted all from 2002-2007 up.