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# Aqa chem 4/ chem 5 june 2016 thread

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Why bother with a post grad course - waste of time? 17-10-2016
1. for the pressure question on EMF i said that there's no change because there is equal number of moles on both side? :/ 100% wrong though, aren't i?
2. (Original post by koolgurl14)
What was the question asking for HCL?
The reagents to go from Cr3+ to Cr2+.
3. Ratio of C2O4^2- to MnO4^1 was 2:5,

There were 26.4cm3 of 0.02 moldm3 MnO4- = 0.000528 mols MnO4 therefore there was 0.00132 moles C2O4^2- in 25cm3 therefore 0.0132 moles in 250cm3,

There was 3 C2O4^2- per one mole of the salt therefore there were 4.4x10-3 moles salt

Mr K3(Fe(c2o4)3).3h20 = 491.1, Mass = moles x mr = 4.4x10-3 x 491.1 = 2.16g

Original mass = 2.22 therefore 2.16/2.22 * 100 = 94.4% purity.
4. (Original post by Hopefulmedic15)
Yeah I didn't resit chem 4 this year so I don't have that to compare to, and it's a bit inaccurate to compare chem 5 to chem 4 grade boundaries because they're completely different papers.
Yep, hopefully boundaries for unit 5 will stay roughly same. I need an A* in this paper atleast to make up for chem 4, Or else I will have to resit next year 😂😂😂😂

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5. What did everyone get for delta H and delta S and for the solution enthalpy question?
6. (Original post by vinchy)
delta G got -65.1
I did but I got it wrong because I said K = 298 + 50 which is 348 instead of 273 + 50!
7. (Original post by DDan123)
I wrote this too, but everybody saying it was increased

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The overall equation is:

H2 + 1/2 O2 ----> H2O

imcreasing conc of either H2 or O2, or increasing pressure, would all shift the equilibrium to the RHS, so EMF increases
8. Just to clarify - for the enthalpy and entropy calculations you had to take into account standard conditions. So you should have divided the equation by 2 to get so2 + 1/2o2 ----> so3 as it is the formation of 1mol of a compound. Then work it out
9. (Original post by Maki.98)
What did everyone get for delta H and delta S and for the solution enthalpy question?
Delta H = -98
Delta S = - 94.5
(divide your values by 2 because it said STANDARD enthalpy and question was for 2 moles)
Enthalpy of solution = -155
10. (Original post by ssamarai)
Said something very similar
(Original post by Fibsy)
It was 3 marks so I put:

MnO4- ions are purple
Filter light so only frequencies absorbed by the sample are let through.
The more concentrated the solution is, the less light it will let through.
Plot calibration graph with known concentrations and their absorbances. Use line of best fit to find concentration.

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WHAAAT! there was a backpage? i Don't remember doing this question, wasn't the 94% answer the final question on the paper?
11. (Original post by Aerosmith)
Just to clarify - for the enthalpy and entropy calculations you had to take into account standard conditions. So you should have divided the equation by 2 to get so2 + 1/2o2 ----> so3 as it is the formation of 1mol of a compound. Then work it out
oh for ****s sake, completely forgot
12. (Original post by RME11)
Ratio of C2O4^2- to MnO4^1 was 2:5,

There were 26.4cm3 of 0.02 moldm3 MnO4- = 0.000528 mols MnO4 therefore there was 0.00132 moles C2O4^2- in 25cm3 therefore 0.0132 moles in 250cm3,

There was 3 C2O4^2- per one mole of the salt therefore there were 4.4x10-3 moles salt

Mr K3(Fe(c2o4)3).3h20 = 491.1, Mass = moles x mr = 4.4x10-3 x 491.1 = 2.16g

Original mass = 2.22 therefore 2.16/2.22 * 100 = 94.4% purity.
im pretty sure that the original mass was 2.29
also how many marks do u think ill get if i got up to 0.0132 right and then carried the error
13. (Original post by OloMed)
The overall equation is:

H2 + 1/2 O2 ----> H2O

imcreasing conc of either H2 or O2, or increasing pressure, would all shift the equilibrium to the RHS, so EMF increases
Bingo!

(Original post by Aerosmith)
Just to clarify - for the enthalpy and entropy calculations you had to take into account standard conditions. So you should have divided the equation by 2 to get so2 + 1/2o2 ----> so3 as it is the formation of 1mol of a compound. Then work it out
Finally someone who agrees.
14. (Original post by Agent R5)
WHAAAT! there was a backpage? i Don't remember doing this question, wasn't the 94% answer the final question on the paper?
Only worth 3 marks.
15. (Original post by K2Cr2O7)
Yes, i think it is.
Thank u. Not sure whether they asked for X(g) or Y(s)? I remember putting CO2 as an answer
16. (Original post by OloMed)
The overall equation is:

H2 + 1/2 O2 ----> H2O

imcreasing conc of either H2 or O2, or increasing pressure, would all shift the equilibrium to the RHS, so EMF increases
Ah, do you remember how many marks that was worth??

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17. For the colorimetry question I put this but am not sure if it is correct.

1. Obtain solutions of Fe2+ at different concentrations and add these solutions to MnO4- one by one.
2. Keep total volume of resultant solution the same by adding H2O as necessary
3. Use colorimeter to detect when resultant solution turns just purple/pink due to excess Mn2+
4. In this solution the ratio of Fe2+ moles to MnO4- is 5:1.
5. Use the moles and concentration of Fe2+ to work out the moles and concentration of MnO4-
18. (Original post by Agent R5)
WHAAAT! there was a backpage? i Don't remember doing this question, wasn't the 94% answer the final question on the paper?
Yeah there was one more question after that
19. (Original post by aerosmith)
just to clarify - for the enthalpy and entropy calculations you had to take into account standard conditions. So you should have divided the equation by 2 to get so2 + 1/2o2 ----> so3 as it is the formation of 1mol of a compound. Then work it out
what??? Ffs
20. (Original post by RME11)
Ratio of C2O4^2- to MnO4^1 was 2:5,

There were 26.4cm3 of 0.02 moldm3 MnO4- = 0.000528 mols MnO4 therefore there was 0.00132 moles C2O4^2- in 25cm3 therefore 0.0132 moles in 250cm3,

There was 3 C2O4^2- per one mole of the salt therefore there were 4.4x10-3 moles salt

Mr K3(Fe(c2o4)3).3h20 = 491.1, Mass = moles x mr = 4.4x10-3 x 491.1 = 2.16g

Original mass = 2.22 therefore 2.16/2.22 * 100 = 94.4% purity.
I got this after a while of crossing most of my working out. The divide by three was the worst part - i kept getting answers more than 100% .

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