Paper 3 (7405/3) AQA Chemistry A-level unofficial markscheme -Grade Poll- 27th June

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Poll: Grade boundary for an A (out of 300)
255 (85%) (21)
5.2%
240 (80%) (40)
9.9%
225(75%) (86)
21.29%
210(70%) (136)
33.66%
195(65%) (75)
18.56%
180(60%) (46)
11.39%
techcloud14
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Would appreciate rep please!!
6 marker----
Add 50cm3 of water to a beaker and measure temperature at regular intervals.Then add MgCl2 at keep measuring temperature at given intervals You had , then calculate the temperature change from extrapolating from a graph of time vs temperature. Then use q=mc </ T to calculate enthalpy change , then divide by moles of MgCl2 which they given you the mass of.
For the molecule with no stereoisoomer=1,3dichloropropane
Also for the mass spectrum multiple choice= butanoic acid
propene. It asked you to do the mechanism producing the major product so the O was attached to the middle C and it asked you why this was formed as the major product (formed from a secondary carbonation intermediate which is more stable). It also asked you to draw the minor product which had the O attached to the end C

entropy -167

6 marker-------
* Dissolve solid in minimum amount of hot solvent to obtain saturated solution in a flash
* Heat allows recrystallization upon cooling.
* Hot filtration using filter paper to remove insoluble impurities
* Place flask in ice to increase yield and form crystals
* Vacuum filtration under reduced pressure
*Dissolve in water to remove soluble impurities


but-1-ene
Z but-2-ene
E but-2-ene

Egg smell-H2S
black solid-Iodine
yellow solid- sulfur

C4H8O impurity was due to butanone

Conc of HX was 0.096, pH was 2.58.
-116 kj mol^-1 for the enthalpy of solution ?
yield 85.3%
multiple choice


ACCCBBCDABBDCACDBBBBBDBABCDCBC



Fox Corner
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Therealbro
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anyone get 104% for the yield
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Whskdjwpq
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(Original post by Therealbro)
anyone get 104% for the yield
Yep, didnt make any sense
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techcloud14
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(Original post by Therealbro)
anyone get 104% for the yield
lool I don't mean to be rude but how is that even possible?!
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Whskdjwpq
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Did anyone get 0.153 for the conc and 2.70 for the pH of the acid?
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techcloud14
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(Original post by Whskdjwpq)
Did anyone get 0.153 for the conc and 2.70 for the pH of the acid?
I got 2 point something too
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Therealbro
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It's an experimental question surely he must have had impurities present.
(Original post by techcloud14)
lool I don't mean to be rude but how is that even possible?!
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techcloud14
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(Original post by Therealbro)
It's an experimental question surely he must have had impurities present.
You're right it is possible.
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computingtrash
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(Original post by Therealbro)
anyone get 104% for the yield
yeah i kept getting 4.02 moles of product out of 4 moles of reactant so i decided to find the percentage yield of the mass and i ended up getting 85.3%. Dunno if its right lmao...
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Flames786
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I wrote about recrystallization for the 6 marker? I hope it was right
* Dissolve solid in minimum amount of hot solvent to obtain saturated solution in a flash
* Heat allows recrystallization upon cooling.
* Hot filtration using filter paper to remove insoluble impurities
* Place flask in ice to increase yield and form crystals
* Vacuum filtration under reduced pressure
*Dissolve in water to remove soluble impurities
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Shannonvaleriex
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(Original post by computingtrash)
yeah i kept getting 4.02 moles of product out of 4 moles of reactant so i decided to find the percentage yield of the mass and i ended up getting 85.3%. Dunno if its right lmao...
I got 85.3%
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alexriversxx
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(Original post by computingtrash)
yeah i kept getting 4.02 moles of product out of 4 moles of reactant so i decided to find the percentage yield of the mass and i ended up getting 85.3%. Dunno if its right lmao...
I got that as well
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Gogregg
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(Original post by Pentaquark)
5.12g/6.00g ×100 = 85.3 percent
Do you mind me asking how you got 6g expected?

Noone at my school could get it right? We had a one to one molar ratio, which ended up with the expected mass being lower than what they actually got?
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Therealbro
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Anyone get -116 kj mol^-1 for the enthalpy of solution ?
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lisjdb
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(Original post by whskdjwpq)
did anyone get 0.153 for the conc and 2.70 for the ph of the acid?
yes got both these
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Jackmapperley
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could impurities in the end product not have cause the yield to be over 100% as it didn't say 5.15g of pure benzoic acid?
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lisjdb
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(Original post by Therealbro)
Anyone get -116 kj mol^-1 for the enthalpy of solution ?
I did -31 - the other number so got a positive i think?? not sure tho
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swilson98
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I got a theoretical yield of 4.88g, making the percentage yield 104% - Due do the benzoic acid not being dry when the mass was measured, hence above 100% yield. I thought AQA were trying to throw us off, but could be wrong...
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Therealbro
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(Original post by swilson98)
I got a theoretical yield of 4.88g, making the percentage yield 104% - Due do the benzoic acid not being dry when the mass was measured, hence above 100% yield. I thought AQA were trying to throw us off, but could be wrong...
That's what I did too
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luliloola
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what did everyone get for the entropy graph question? that really stumped me
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