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

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Reply 180

TheNote

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

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Reply 181

TheNote

Original post by tasniaa

jihadbexdnw feih
3 marks down the drain.

I think I've got 48/60 :frown:

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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Reply 182

madrevision

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

Reply 183

TheNote

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

(edited 10 years ago)

Reply 184

Kathiye

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

Reply 185

Izzy_saul

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.

Reply 186

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..

Reply 187

krisshP

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

Reply 188

madrevision

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 :smile:

Reply 189

krisshP

What did everyone put for the neutralisation question and how it does more neutralisation?
I said it's got and extra OH-

Reply 190

madrevision

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

Reply 191

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.

Reply 192

krisshP

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

Reply 193

krisshP

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

Reply 194

Kathiye

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.

Reply 195

nick d terrier

Any news of an unnofficial mark scheme??

Reply 196

username638250

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

Reply 197

CoolRunner

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)