Cssa chemistry 2012 (3 Viewers)

[get_back23]

how about the titration question? i want to kno

did every1 get a answer smaller than 1 for concentration? the vinegar one, coz a lot of ppl got 1.something, so i was like 0_0

 

[TheBloop]

I remember that question,
It said something about which one would cause the greatest pH change if one of the following was added to HCl
I was tossing between diluting it with water and adding NaOH ...

Case with diluting it with water:
1L of dilute HCl: 0.001M = 0.001mol.L^-1
pH = -log₁₀(0.001) = 2

Add 1L of water:
c = n/v = 0.001/(1+1) = 0.0005mol.L^-1

Adding 1L of 1M NaOH:
1M = 1mol.L^-1
Therefore, 1mol of NaOH.
pH = 14 - (-log₁₀(1)) = 14 - 0 = 14

Adding this to HCl:

HCl_(aq) + NaOH_(aq) --> H₂O + NaCl_(aq)

Limiting agent is HCl.
Therefore NaOH remaining at the end is:
1mol - 0.001mol = 0.999moles of NaOH and 0 moles of HCl
Everything else is neutral.
Therefore, concentration is:
c = n/v = 0.999mol/2L = 0.4995M

pH = 14 - (-log[0.4995]) = 13.698… = 13.7

Therefore, adding NaOH brings the greatest change here.

 

[tickleyouemo]

how about the titration question? i want to kno

did every1 get a answer smaller than 1 for concentration? the vinegar one, coz a lot of ppl got 1.something, so i was like 0_0

Do you remember the question? i want to work it out again, because my friend got an answer smaller than 1 but i got 1. something ....

 

[Kimyia]

Did anyone get that sodium iodide/fluoride question?? i was so stuck with that one

 

[golgo13]

Guys just remember the marking is based on your teachers discretion, although there might be a marking criteria its really based on your teacher

 

[blieu]

Guys just remember the marking is based on your teachers discretion, although there might be a marking criteria its really based on your teacher

I hope my teacher gives me marks for trying LOL

 

[golgo13]

Worse comes to worse ask for the criteria and try to point out that u had what they were looking for in the answer XD

 

[habitres]

Didn't the question change to sodium fluoride?

Haha. Yes! it got changed to NaF(s)
Our teacher actually picked that up like one or two days ago and emailed/rang the CSSA and made them change it/edit it as NaI is neutral and doesn't work with the question.

Did anyone get that sodium iodide/fluoride question?? i was so stuck with that one

Yea, I got it (pretty much)
Let me try and explain.
NaF is a basic salt. The reaction (equilibrium) is:

NaOH + HF <-> NaF + H2O

When excess NaF is added (20g) it moves to the left according to Le Chatelier's Principle, and hence it becomes basic as NaOH is a strong base (HF is a weak acid)

The ionisation reaction involved OH- and H3O but I couldn't/haven't been bothered to work it out. Pretty much that the Na doesn't have anything to do with the actual reaction.

 

[inJust]

Such an easy exam, expecting full marks.

 

[barbernator]

Such an easy exam, expecting full marks.

inb4 you don't get full marks.

 

[jeffwu95]

we got it changed it C6H8O7, didnt realise it was citric acid );

 

[DrWho94]

The multiple choice was killer>

 

[DrWho94]

Oh and please tell me the NaI question is wrong...I was thinking that it was neutral so the question totally didn't make sense...

 

[TheBloop]

we got it changed it C6H8O7, didnt realise it was citric acid );

Devo…

Should've attempted to draw it ad realised it didn't much up, and then been like "wait… extra hydrogens!"

 

[vinhkn]

OH FUCK I CHANGED MY ANSWER FROM NAOH TO DILUTING IN THE LAST MINUTE OF THE EXAM FUCKKKKK, it's gonna cost me a 90 now

 

[Jason Xie]

who remember the 3rd multiple choice questions in CSSA Chem paper and what did say? from Ni3+ to Ni2+ something like that

 

[Sindivyn]

who remember the 3rd multiple choice questions in CSSA Chem paper and what did say? from Ni3+ to Ni2+ something like that

That's what I had.

 

[habitres]

yea same here. it was D.

 

[HeyLook]

I just done the cssa chemistry, do u have a copy of it? if you do, can u please send it to me? please! thanks!

 

[vinhkn]

How'd everyone go?