Chemistry Predictions/Thoughts (1 Viewer)

[BionicMango]

that would be evil if they made us do that though... would you just use henderson hasselbach for every conjugate pair and sum up the concentration of protons? that feels dodge as i dont think so...

Not in the hsc but as far as I know each dissociation DOES have its own Ka value so theoretically you would? But again not in the hsc and if they did then they would give you the formula and and a decent number of marks

 

[killer queen]

ohh wait we can just do that? does that give the ph?

Uh I don’t remember sorry but that question was polyprotic, if you have the time plz check if that’s what you’re looking at? I just remember there being some weird way of doing it

 

[BionicMango]

I remember figuring out how to do Ka stuff with polyprotic acids but I don't remember anymore, something to do with multiplying all the individual Kas equalling the total Ka (giving related rates ahaha) but unfortunately I don't remember

Write down the equation as if it dissociates twice (so H2A + 2H2O <—> A^2- + 2H3O^+). Then write KA (ignore H2O) remembering to square the terms with the two because of the molar ratio, and solve for [H3O^+] like normal ([H3O^+] = 2[A^2-] as well)

 

[Trial&Error]

that would be evil if they made us do that though... would you just use henderson hasselbach for every conjugate pair and sum up the concentration of protons? that feels dodge as i dont think so...

No for polyprotic acids there's the possibility of having 2 buffer regions. Have a look at the titration curve for it. So depending on the question you'll have one of those buffers and you apply the henderson-hasselbach equation only once for it.

For example for H2CO3 the first buffer is: H2CO3 ⇋ H+ + HCO3− (Ka1) ---> for this use [H2A] and [HA-]
And the second buffer is: HCO3− ⇋ H+ + CO32− (Ka2) ---> for this use [HA-] and [A2-]

Both of these buffers have different Ka and hence different pKa which I think generally will be given.
so the question will tell you which specific buffer is being used, because they obviously won't be using them both at once.
Then you find [HA] and [A-] according to that.

I really hope this doesn't come up though cause it's even harder when they do it in terms of a weak base and its conjugate acid.

 

[Trial&Error]

I remember figuring out how to do Ka stuff with polyprotic acids but I don't remember anymore, something to do with multiplying all the individual Kas equalling the total Ka (giving related rates ahaha) but unfortunately I don't remember

edit: is q36 of 2021 what y'all are talking about?

For this question the equations 1 and 2 are like stepwise equations of the net equation 3.
So if you remember in yr 11 we learnt how to add equations up by cancelling common reactants and products. That's essentially the same principle applied to the Ka formula.

So if you write the Ka expressions for equations 1, 2 and 3 you'll find that in order to get 3, you have to multiply 1 and 2 so the common products and reactants cross out.

Hope this helps

 

[tsswhzl]

Has anyone seen the questions which can be asked about Carbonic acid equilibria in blood. There is a chance it comes up. Tricky question. I was the only one to score full marks in my cohort on it. I can attach it if someone wants with solutions.

 

[nowomansland]

Has anyone seen the questions which can be asked about Carbonic acid equilibria in blood. There is a chance it comes up. Tricky question. I was the only one to score full marks in my cohort on it. I can attach it if someone wants with solutions.

ooh do share!

 

[tsswhzl]

ooh do share!

Okay ill also add any other weird q’s on trial

 

[t234]

Also as far as I know hendersen hesselbach doesn’t work for multiprotic acids/bases? Only monoprotic

wait really?

 

[tsswhzl]

ooh do share!

Okay here is the question about carbonic acid in blood and another equilibrium system ( download them to avoid breaking your neck )

View attachment IMG_0311.jpegView attachment IMG_0312.jpeg
Here is a question about soap ( you probably already know this one ) I can transcribe this if needed
View attachment IMG_0310.jpeg

 

[nowomansland]

Okay here is the question about carbonic acid in blood and another equilibrium system ( download them to avoid breaking your neck )

View attachment 50146View attachment 50147
Here is a question about soap ( you probably already know this one ) I can transcribe this if needed
View attachment 50145

the second part is def quite weird but still doable. i doubt id get full marks tho bc it seems to require clear explanation with link to info

 

[nowomansland]

also why is chem alignment kinda low compared to subjects like maths adv (which is relatively easier but has better alignment)? is it bc of the cohort?

 

[retroojh]

coz its ez lmao

 

[coolcat6778]

coz its ez lmao

math advanced is way easier... yet u only need a 76 for a band 6. compare that to chem 87

 

[tsswhzl]

also why is chem alignment kinda low compared to subjects like maths adv (which is relatively easier but has better alignment)? is it bc of the cohort?

yeah true adv is quite a bit easier + you dont need any memory or studying in advanced only practice, but i think if people spent as much time on chem as maths the average mark would go up a bit

 

[coolcat6778]

yeah true adv is quite a bit easier + you dont need any memory or studying in advanced only practice, but i think if people spent as much time on chem as maths the average mark would go up a bit

As average mark goes up, alignment becomes worse (higher mark required for band 6).

NESA is purposely doing this to make sure only 10% of the state gets a band 6 in chem. In Advanced 22% of the state gets a band 6. They make the sciences band 6 rates so low for some reason

 

[tsswhzl]

As average mark goes up, alignment becomes worse (higher mark required for band 6).

NESA is purposely doing this to make sure only 10% of the state gets a band 6 in chem. In Advanced 22% of the state gets a band 6. They make the sciences band 6 rates so low for some reason

yeah i was realising that as i typed, the exams would just get even harder. tbh it is probably because maths is 'hard' and 'evil' , and it is generally thought 'you cant get better at maths', 'its a different language', when advanced is just regular school maths in europe and asian schools

 

[reniiiblaseee]

chemistry lowkey scales like shit i thought it would weigh more tbh

this means the big three are physics, math ex1 and math ex2

 

[coolcat6778]

chemistry lowkey scales like shit i thought it would weigh more tbh

this means the big three are physics, math ex1 and math ex2

hsc physics somehow scales and aligns so well despite it being a humanities exam lol. hsc chemistry is more cognitively demanding

 

[reniiiblaseee]

physics somehow scales and aligns so well despite it being a humanities exam lol. chemistry is more cognitively demanding

how tf is it humanities