I divided the masses by the relative atomic masses to get the moles. I then multiplied the moles by 1000 to get a whole number of 30 and 45. Then I said the empirical formula was Pb30O45, but I didn't realise I could simplify it to Pb2O3 :frown:

Stupid mistake.

Reply 25

_Charlotte15

17

Original post by spnlove

Don't worry that was basically the only question I could do! I messed up sooo many!

Same it was so difficult!!

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Reply 26

alexhambrook

Original post by spnlove

You have to double 1 and 1.5 so that it's the same ratio but there are no decimals.

I spoke to my chem teacher post-exam, you do have to double it to get integers, I forgot and only used 1:1.5 but i'm sure we'll get method marks

Reply 27

mkea

Original post by _Charlotte15

That might be right, but I got the ratio 1 and 1.5 so you have to round them. Where did the 2 and 3 come from?

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Wasn't it something like 6.61/207 for Pb and 0.72/16 for 02
so thats 0.0319323671 for Pb and 0.045 for 0xygen
0.045 / 0.030
so ratio of 2:3 i think?! Or something like that, i'm not 100% sure!

Reply 28

Bailey280899

1

Original post by mkea

Wasn't it something like 6.61/207 for Pb and 0.72/16 for 02
so thats 0.0319323671 for Pb and 0.045 for 0xygen
0.045 / 0.030
so ratio of 2:3 i think?! Or something like that, i'm not 100% sure!

Empirical formula is the simplest (whole number) ratio. So I multiplied it by 1000. Not sure if that's right, but we'll see.

Reply 29

emma0_0star

Original post by _Charlotte15

That might be right, but I got the ratio 1 and 1.5 so you have to round them. Where did the 2 and 3 come from?

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You can't round numbers up when you find the empirical formula as you have to find the lowest common factor (i think that's right, i dunno, something like that) between both of them. Because you can't put Pb1O1.5 you have to multiply the two numbers by two so that 1.5 becomes a whole number. You can't round it because what you do to one number, you have to do the same to the other. Hope that kind of helps :/

Reply 30

_Charlotte15

17

Original post by alexhambrook

I spoke to my chem teacher post-exam, you do have to double it to get integers, I forgot and only used 1:1.5 but i'm sure we'll get method marks

Yes surely we'll get some marks because we were half way there. How many marks was it worth again, 2?

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Reply 31

LukeMontgomery

5

Original post by _Charlotte15

That might be right, but I got the ratio 1 and 1.5 so you have to round them. Where did the 2 and 3 come from?

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Basically, you have to make the two numbers whole, and as the numbers are 1 and 1.5, the easiest way to do this is to double them both, making it 2 and three, hence the answer is Pb2O3. You don't round as what you do to one number you have to do with the other number.

Reply 32

_Charlotte15

17

Original post by emma0_0star

You can't round numbers up when you find the empirical formula as you have to find the lowest common factor (i think that's right, i dunno, something like that) between both of them. Because you can't put Pb1O1.5 you have to multiply the two numbers by two so that 1.5 becomes a whole number. You can't round it because what you do to one number, you have to do the same to the other. Hope that kind of helps :/

Oh god yeah :frown:

which I'd have seen this last night haha.

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Reply 33

_Charlotte15

17

Original post by LukeMontgomery

Basically, you have to make the two numbers whole, and as the numbers are 1 and 1.5, the easiest way to do this is to double them both, making it 2 and three, hence the answer is Pb2O3. You don't round as what you do to one number you have to do with the other number.

Yes I understand now, wasted an easy two marks. Thank you!!

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Reply 34

LoganF

OP

2

Original post by Olivia197

What did you put for explaining the 6 mark question on C2 and the 5 mark question on C3 about the two methods?

For the C2 paper I put: as concentration increases, the temperature change also increases and I explained collision theory. Then I said the temperature change stopped at 0.7 moles and it was constant because the rate of reaction stopped and reactants stopped reacting.
For the C3 paper, the 5 mark question was difficult. So I made some stuff up about: From the precipitate you can weigh it so it was more accurate. And for the titration method I said it is accurate as it tells you exactly how much acid you need. And I made a few other points.
What did you put for those questions?

Reply 35

LoganF

OP

2

Original post by RachJess4

I thoguht C3 was a pretty good paper! Most questions were just reproducing information from the spec rather than having to explain stuff which was awesome! I'm pretty sure I failed the 5 marker on the different methods of finding the concentration of a substance... I couldn't think of anything! I know I made a LOT of silly mistakes in C2 though :/

Yes I'm the same, I made silly mistakes in both papers. For example for the empiracal formula question I read the calculater wrong, I read it as 15, when it was 1.5, so I'm annoyed at that.

Reply 36

hoorya

what was the ethical issues for putting iodine in people's diet???
and how did u balanced the equation??

Reply 37

LoganF

OP

2

Original post by hoorya

what was the ethical issues for putting iodine in people's diet???
and how did u balanced the equation??

For the ethical issues, I said that it was peoples choice if they wanted iodine in their diet.
For balancing equation I put a 2 on the reactants second part

Reply 38

hoorya

what about the questions said tick two boxes?
i said the 2nd and the forth one?

Reply 39

*SweaterWeather*

6

Original post by LoganF

For the C2 paper I put: as concentration increases, the temperature change also increases and I explained collision theory. Then I said the temperature change stopped at 0.7 moles and it was constant because the rate of reaction stopped and reactants stopped reacting.
For the C3 paper, the 5 mark question was difficult. So I made some stuff up about: From the precipitate you can weigh it so it was more accurate. And for the titration method I said it is accurate as it tells you exactly how much acid you need. And I made a few other points.
What did you put for those questions?

I put the same for the 6 marker, then for the 5 marker i said that when weighing you could lose some of the product during filtration/drying so this would make the results less valid/incorrect, but since titrations are normally done to the nearest cm3 the resolution is low so this would affect the calculation of the concentration? I also said that titrations would have to be repeated a few times with the indicator to create a mean so any anomalies wouldnt affect it when you dont have the indicator so this'd take quite a long time to do :/

but the some of the questions were really confusing, both papers were nothing like i expected :redface:

C2 & C3 - AQA GCSE CHEMISTRY 14th May

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