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# OCR B (Salters) Chemistry F331 (Chemistry for Life) Thursday 23rd May Exam Discussion Watch

1. It was, .2/84 =y
Y= co2 mole
Y x24 x1000

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2. (Original post by tasniaa)
3 marks down the drain.

I think I've got 48/60
Don't think that'll be good enough for an A
Damn :'(
An A is normally 48, this was easier so maybe 50
Plus grades don't matter at this stage

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3. (Original post by Ikklemini)
Urm I wrote about using limewater and testing the time it takes for it to go milky, the longer time the higher the thermal stability- bot sure if thats what they asked for- also said Sr is more thermally stable than Mg as thermal stability of Carbonates increases down group 2

What did everyone else write ?
yh that's right but I got confused because before the exam I read something where it was Ca and Mg so I ended up writing it would take longer for the Ca solution to turn cloudy, but I got everything else so I should get about 2/4
4. (Original post by CoolRunner)
I got 18.000 for number of years about the half life question

I also got 56.9 for the volume in cm3
How did you calculate the half life?
I did 6000 * 0.875 to give 5250 years
5. (Original post by TheNote)
How did you calculate the half life?
I did 6000 * 0.875 to give 5250 years which fits for the ice age stuff they talked about.
You divide 100 by 2 = 50
/2 = 25
/2 = 12.5

Since it has halved 3 times, there have been 3 half lives. 3 x 6000 = 18,000
6. (Original post by TheNote)
How did you calculate the half life?
I did 6000 * 0.875 to give 5250 years

As in the question it said they found 12.5% remaining and the half life is 6000 years.

I did 100% to 50% is 6000 years
50% to 25% is 6000 years
25% to 12.5% is 6000 years

3 amounts of 6000 years had gone by so 18,000 years, as it is decay you start with the full amount, halve that, then halve that etc.
7. For the gas question, I left it as 56.88 as it didn't specify a certain amount of significant figures, will I get marked down..
8. (Original post by Ikklemini)
Hey what did everyone get for the years from the half life question? I got something like 26000 years ?!
1 HF-->50%-->6000 years
2HF-->25%-->12,000 years
3HF-->12.5%-->18,000 years

They told you the HF was 6000 years
9. (Original post by Izzy_saul)
For the gas question, I left it as 56.88 as it didn't specify a certain amount of significant figures, will I get marked down..
I think it should be fine 2dp is not an unreasonable degree of accuracy
10. What did everyone put for the neutralisation question and how it does more neutralisation?
I said it's got and extra OH-
11. (Original post by krisshP)
What did everyone put for the neutralisation question and how it does more neutralisation?
I said it's got and extra OH-
yh I put it's got more OH- ions, these are electron donors lol
12. What everyone say foe the question on why there's a smaller peak at m/z 20?
I threw in molecular fragmentation of water.
13. (Original post by anthonyrf)
I calculated the Mr of MgCO3 which was 84.3 I think.

The question told you that we had 0.2g of MgCO3 so the no. Moles of MgCO3 was 0.2/84.3 = 0.00237247...

ratio of CO2 to MgCO3 was 1:1 so moles of CO2 = 0.00237247...

therefore Vol. of CO2 in cm3 was 0.00237247... x 24000 which = 56.94 (2dp)
Yay that's what I did
14. Wait say if in the calculation for CO2 volume in cm3 I used accurate values ie fraction for the moles of CO2/magnesium carbonate, but I did round my final answer to 3SF and was right, would I get full marks for the question?

Thanks
15. (Original post by krisshP)
What everyone say foe the question on why there's a smaller peak at m/z 20?
I threw in molecular fragmentation of water.
I thought that the question there was about oxygen... did I read it wrong? x.x Oh no! I get it now. Looks like that's another mark that I lost... I assume that it was an ion where the oxygen had an atomic mass of 18.
16. Any news of an unnofficial mark scheme??
17. Oh no, I said for the one on the last page)
H2O+
and 'Water contains an isotope of water which has a mass of 16'
ugh
18. I'm such an idiot.

For the approximation/assuming enthaply change question i written this, will i got 2 marks?

1) Hcl has the same density as 1gcm-1 (SAME AS WATER)
2) Hcl has a specific heat capacity as 4.18gj-1k-1 (same as water)

do i get the marks, even though my values are stupidly wrong (i did put brackets to say it is the same as water for both)
19. how many marks was the half life question and the table with CO2 and Sio2 and diamond
20. (Original post by emicroteen96)
how many marks was the half life question and the table with CO2 and Sio2 and diamond
1 mark? I think.

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