STAV 2008,2009,2010

[Cuntryboner]

If you already have access to these, don't bother, If you don't, enjoy!
http://www.mediafire.com/?sharekey=f749306f1be700dc5bf1f12f1ff3f30a7ea83e5bb1b321f292595bc19e6628dc

[Martoman]

oh. Karma for you for 2008. Good job!

These are hard to find because of its copyright whinging that the company has.

[azn_dj]

Question about Question 20 in STAV 2009

Question 20
In a titration of a strong base with a strong acid, the following procedure was used
1. A burette was rinsed with water and then filled with the standardised acid.
2. A pipette was rinsed with some base solution.
3. A conical flask was rinsed with some base solution.
4. A pipette was used to transfer a measured volume of base solution into the conical flask.
5. Indicator was added to the base sample and it was titrated to the endpoint with the acid.
Which of the following statements is correct?
A. The calculated base concentration will be correct.
B. The calculated base concentration will be too low.
C. The calculated base concentration will be too high.
D. No definite conclusion can be reached about the base concentration.


The Answer:
Question 20
The conical flask should not have been washed with base solution but with de-ionised water. More acid
will be needed. The number of mole of base will be higher and therefore its concentration will appear
higher. Correct answer is C.

But I dont agree with this. When you look at these two points.....
1. A burette was rinsed with water and then filled with the standardised acid.
3. A conical flask was rinsed with some base solution.
Hence shouldnt the answer be D?

[stonecold]

No. They are so going to put a question about this on the exam, as it is so confusing and will get many people. 

Your standardised acid (i.e. known concentration) has been diluted, and on top of that, the conical flask now has more mol of NaOH in it.  You will therefore require a greater volume of HCl to reach the endpoint.  Then you go to work out you mol of HCl using n=cv.  As volume is bigger, mol will be bigger.

And as mol is now bigger, conc. NaOH will be increased as well.

[m@tty]

Yep, it's C.

[cindyy]

if it is a hard question, wouldnt that just make them wanna put it on the vcaa papers even more so they can seperate us ?

[stonecold]

^isn't that what I said lol

[m@tty]

I would love a question like this... If you look at it methodically, trying to spot what's out of the ordinary, then logically deduce how this will affect the average titre. Then they're actually quite easy.

[happyhappyland]

thanks!!! i only needed 08 though

[stonecold]

Yeah, but I reckon they might give us a reverse one, where an increased volume results in a lower concentration...  like one where the unknown is in the burette.

[physics]

This doesnt fit herre but i was raging at the fact that TSSM makes trial papers after all it stands for The specialised school for mathematics =.= and i got a lot wrong D:

[cindyy]

^isn't that what I said lol

HAHA shh misread it!

[azn_dj]

Ouch - yeah both increase calculated concentration. Doh!
So thats 3 marks gone over the whole paper.
But that is certainly a very trick question. At least its better to make all these mistakes now than later? =D

[Richiie]

Thank you!