Competition - post a photo you've taken of nature >>

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

This poll is closed

Grade boundary for an A (out of 300)

255 (85%) 5%

240 (80%) 10%

225(75%) 21%

210(70%) 34%

195(65%) 19%

180(60%)11%

Total votes: 404

techcloud14

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

(edited 6 years ago)

Scroll to see replies

Reply 1

Therealbro

anyone get 104% for the yield

Reply 2

Whskdjwpq

Original post by Therealbro

anyone get 104% for the yield

Yep, didnt make any sense

Reply 3

techcloud14

Original post by Therealbro

anyone get 104% for the yield

lool I don't mean to be rude but how is that even possible?!

Reply 4

Whskdjwpq

Did anyone get 0.153 for the conc and 2.70 for the pH of the acid?

Reply 5

techcloud14

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

Reply 6

Therealbro

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?!

Reply 7

techcloud14

Original post by Therealbro

It's an experimental question surely he must have had impurities present.

You're right it is possible.

Reply 8

computingtrash

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...

Reply 9

Flames786

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

Reply 10

Shannonvaleriex

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%

Reply 11

alexriversxx

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

Reply 12

Gogregg

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?

Reply 13

Therealbro

Anyone get -116 kj mol^-1 for the enthalpy of solution ?

Reply 14

lisjdb

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

Reply 15

Jackmapperley

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?

Reply 16

lisjdb

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

Reply 17

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...