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# Aqa chem 4/ chem 5 june 2016 thread Watch

1. (Original post by lahigueraxxx)
Why does the molecular ion have the same mr as the compound?? Surely if it's an ion and there is one electron knocked off, its mr should be lower?
The mass of an electron is tiny!!
2. (Original post by lahigueraxxx)
Why does the molecular ion have the same mr as the compound?? Surely if it's an ion and there is one electron knocked off, its mr should be lower?
The mass of an electron is pretty much close to 0. mass number is equal to protons + neutrons

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3. Please can you explain q4a Jan -12 I really don't know where the answer came from
4. (Original post by SubwayLover1)
The mass of an electron is tiny!!
Ah fair enough - thanks!
5. (Original post by Limerence.)
The mass of an electron is pretty much close to 0. mass number is equal to protons + neutrons

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Thank you! Makes sense now
6. Hey guys I'm really stuck on how they worked out the moles of so3 to be 5.2( 8-2.8?i guess, I don't know why) but I got the 2.8 for so3.
Questions is this: 2SO3 <--> 2SO2 + O2
At start so3 is 8 moles
At equilibrium O2 is 1.4
Determine amount of so2 and so3 at equilibrium
Usually I don't have any troubles with these, maybe I'm having a mental block, chem4 I'm pretty much sound at everything else. Thanks!
7. Can they ask questions about the AS mechanisms in the unit 4 exam? or will it just be the A2 mechanisms like Nucleophillic addition-elimination?
8. (Original post by emsieMC)
Hey guys I'm really stuck on how they worked out the moles of so3 to be 5.2( 8-2.8?i guess, I don't know why) but I got the 2.8 for so3.
Questions is this: 2SO3 <--> 2SO2 + O2
At start so3 is 8 moles
At equilibrium O2 is 1.4
Determine amount of so2 and so3 at equilibrium
Usually I don't have any troubles with these, maybe I'm having a mental block, chem4 I'm pretty much sound at everything else. Thanks!
You've made 1.4mol of O2 and SO3:O2 is a 2:1 ratio, so 2.8mol of SO3 has been used up. You started off with 8 so 8-2.8.
9. Good luck everyone! Let's smash this
10. (Original post by Matt Johnson)
Can they ask questions about the AS mechanisms in the unit 4 exam? or will it just be the A2 mechanisms like Nucleophillic addition-elimination?
Yes they very much can ask for AS mechanisms like in previous years.

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11. why is it that the larger the ka value the stronger the weak acid?
12. (Original post by Parallex)
You've made 1.4mol of O2 and SO3:O2 is a 2:1 ratio, so 2.8mol of SO3 has been used up. You started off with 8 so 8-2.8.
Thank you but shouldn't it also minus the 2.8 from so2? because 2.8 miles of so2 is also formed
13. (Original post by Lilly1234567890)
why is it that the larger the ka value the stronger the weak acid?
From what I know, I'd suppose it was something to do with the fact that a larger Ka value means higher [H+] and [X-] as they go on the top of the Ka equation and therefore as there is higher concentration of both, the acid will dissociate more thus stronger acid as strong acids fully dissociate
14. http://filestore.aqa.org.uk/subjects...4-QP-JUN12.PDF

3bi can someone explain?
15. (Original post by emsieMC)
Thank you but shouldn't it also minus the 2.8 from so2? because 2.8 miles of so2 is also formed
For every 2mol of SO3 that breaks down, 2mol of SO2 and 1 mol of O2 are formed. We're not saying that some SO3 breaks down to form SO2 and some breaks down to O2, it's that each mole of SO3 that breaks down forms one mole of SO2 AND half a mole of O2.
16. (Original post by Lilly1234567890)
why is it that the larger the ka value the stronger the weak acid?
Ka = (h+)((x-)/(hx) so Ka is proportional to h+ which higher in a stronger weak acid

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17. H2O is protonated going from the LHS to the RHS, so it's acting as a base. It accepts a H+ to form H3O+.

Now look at the backwards reaction by itself, CH3COO- accepts a H+ to form CH3COOH. That is also acting as a base as it is being protonated.
18. (Original post by Parallex)
H2O is protonated going from the LHS to the RHS, so it's acting as a base. It accepts a H+ to form H3O+.

Now look at the backwards reaction by itself, CH3COO- accepts a H+ to form CH3COOH. That is also acting as a base as it is being protonated.
So with the second one CH3Nh2 gains a H+ so is an acid? What about the water?
19. (Original post by Super199)
So with the second one CH3Nh2 gains a H+ so is an acid? What about the water?
CH3NH2 is the base because it accepts a H+

H2O loses a proton in the forward reaction so it's acting as an acid.
20. (Original post by Parallex)

Hopefully that's alright for you.
This is great but I lose it when is comes to 1/2 X moles of H+ neutralise 1/2x moles of OH-

would you be able to substitute some made up random simple numbers in it and exemplify?

Thankyou, sorry to be a pain!

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