STAV 2010

[VCE123456789]

Is the answers for question 7d) wrong? I got 98.2%

[stonecold]

yeah, that exam is shit.  it has several wrong answers...

[VCE123456789]

I got told they were one of the best

[Martoman]

LOLi  just was thinking the same.

What else is wrong in it?

[stonecold]

they say di-sulfide is not a covelent bond.  and they ripped this question off from neap.

[Martoman]

*sigh* @ question 7.

A student wishes to determine the concentration of citric acid in fruit juice. The student titrates 20 ml samples of standard NaOH solution with diluted fruit juice from a burette. Four experiments were carried out and the following titres obtained:

19.55 20.35, 20.3 and 20.35

The discrepancy in the first titration could be due to the student washing the:

A) Conical flash with NaOH only.

B) Buerette with water only

C) Pipette with NaOH only.

D) Pipette with water only.


Ok I want to explain my thinking so you guys can point out what i'm thinking wrong.

A) This means more NaOH in the conical flask. If you have more, you need to add more (?!?! i think? I mean it wouldn't matter if there was 100 million grams of NaOH really? it will react the same no?)

B) Means less diluted fruit juice means MORE needed. (still not clear why but it feels right)

C) Is what you should do anyway

D) Less NaOH means more needs to be added. I don't see why not.

[m@tty]

A; that is correct. You have more mole of NaOH so you need to add more acid.

B; again correct. You have less acid in the burette therefore what you shoot out the bottom is less concentrated, it also has less mole. Therefore a higher titre is needed.

C; as you said, this is what you are meant to do.

D; Indeed, there is less NaOH in the conical flask, therefore less acid is required. ie. A lower titre. This is the answer.

[stonecold]

Less NaOH = less mol = smaller titre

[Martoman]

OH LOL

I think i get it.

The conical flask mol is the REFERENCE point.

If that increases so does the other.... similarly with the decrease. RIGHT. OK. 

[kenhung123]

Hmm did you notice the problem is in the small titre? (Not trying to be rude) because less titre means less mole in conical=less mole to react which means something happened caused less mole to be in conical so D

[Martoman]

yeah ken.

Problem is i didn't get these problems until a few seconds ago.

[Martoman]

Also

Which of the following could oxidise ions?

A) H+

B) Fe2+

C) Cu

D) H2O2

The way I thought about it meant that the thing we are looking for is to cause oxidation in the I- meaning it is reducing meaning it is gaining. H+ can gain. Fe 2+ can gain. Cu doesn't like to. h2O2 can.

Now how do i decide? Answers suggest going to the electrochemical series...... but wtf.

[m@tty]

Yep, you have to go to the electrochemical series. Doubt there'd be a question like this in the exam though, but it's still possible. Species higher in the series are stronger oxidants. Also look at the half equations; to oxidise I^- it must be reduced itself. So rule out H+(because it's lower on series) and Cu(because it is lower on series and isn't reduced) automatically. You also rule out Fe²⁺ as it's reduction is lower on series. Then you're left with H₂O₂. This is of course if I remember correctly from last year =\

[Martoman]

lol yeah my lack of 1-2 is shining through. i think i;ll read the 1-2 textbook today