VCE Chemistry Question Thread

[Bestie]

Hello
for the attached question below:
why can't acid base titration be used to determine the amount of aspirin produced?

the ans says: Acid–base titration could not be used as there will be unused salicylic acid in the reaction vessel, resulting in
an incorrect equivalence point.

Why would there be excess?

can someone please explain???

please anyone?

[Robert123]

please anyone?

Ok, so you have an unknown amount of salicyclic acid in the flask. If you add a known amount of base to it, some would react with the salicyclic acid and some with the asprin (I belei asprin does exhibit some acidic properties due to the -COOH group). So you know how much base has reacted but you won't be able to determine howich of it reacted with asprin or howich reacted with salicyclic acid, therefore, you can not determine the amount of asprin produce.
Does that clarify it for you?

thank you robert123

Has anyone done the VCAA sample?
for ii) the ans says
NaOH(aq) + HA(aq) ⇌ NaA(aq) + H2O(l)
n( HA) = n ( NaOH) = 14.4/1000 x 0.100 = 1.44 x 10‐3mol.
Conc. (HA) = 1.44 x 10‐3 x 1000/20.0 = 0.0720M

where they get the 20L?
I got 0.144M?

It's not 20L its 20ml (top right of the page, 20ml aliquot.
The last line of working is pretty messy so I'll try and explain it for you.
C(HA)=n/v
=1.44*10^-3/(20/1000)  (the 20/1000 is to convert ml to L)
Now remember year 9 fractions, when diving by a fraction, you flip it around and multiply it so
C= 1.44*10^-3 x 1000/20 = 0.0720M

What part did you do differently?

[allstar]

thanks again! sorry for some reason my hardcop said 10ml aliquots but the pdf version says 20ml

I have another question if thats okay?

the attached one, i'm not sure how to do and especially the highlighted bit, what do I do about that?

Thank you

[magneto]

for this reaction:
caco3 + 2hcl -> cacl2 +co2 +h2o

which one should be the limiting and which one should be the excess in order to ensure complete reaction?

I need to design an experiment to test the affect of conc on hcl on reaction rate, I ddn't know whether to have caco3 in excess or hcl?

[soNasty]

usually the acid would be in excess, in order to completely react with caco3

[yang_dong]

lisachem 2011 question? shifty ans?

Calculate the average volume, in mL, of the concordant titres of potassium
permanganate solution.
shouldn't it be (18.32+18.07+18.16)/3 not 17.98, 18.07 and 18.16 mL
Average titre = (17.98 + 18.07 + 18.16) / 3
= 18.07 mL

Calculate the concentration of the potassium permanganate solution as a % (m/V).
[i got the working out, but where did they get 197.1g/mol, I thought it would be 54.9 + 64 + 39.1 = 158?
n(KMnO4) in average titre = 3.264x10-5 mol
m(KMnO4) in average titre = 3.264x10-5 mol x 197.1 g mol-1
= 0.06434 g 
% (m/V) KMnO4 = m(KMnO4) / V(average titre) x 100
= (0.06434 / 18.07) x 100
= 0.03561 %

Thank you in advance

[swagsxcboi]

lisachem 2011 question? shifty ans?

Calculate the average volume, in mL, of the concordant titres of potassium
permanganate solution.
shouldn't it be (18.32+18.07+18.16)/3 not 17.98, 18.07 and 18.16 mL
Average titre = (17.98 + 18.07 + 18.16) / 3
= 18.07 mL

I thought for concordant titres, the highest and lowest titres need to be + or - 0.1mL?
Which in both cases it doesn't seem to work out :/

were there any other titres given?

[Yacoubb]

I thought for concordant titres, the highest and lowest titres need to be + or - 0.1mL?
Which in both cases it doesn't seem to work out :/

were there any other titres given?

It could very well be an error to be honest. A lot of trial paper companies seem to be making mistakes in Chemistry exams. It gets pretty annoying, especially when you start to doubt yourself when you get a question wrong you KNOW how to answer.

[swagsxcboi]

It could very well be an error to be honest. A lot of trial paper companies seem to be making mistakes in Chemistry exams. It gets pretty annoying, especially when you start to doubt yourself when you get a question wrong you KNOW how to answer.

I know right 0_0  even VCAA have quite a few trippy mistakes in their assessment report that make you doubt yourself

[Sanguinne]

Why is carbon rod used instead of a copper rod for a half cell containing CuCl2 solution (galvanic)


Thanks

[Yacoubb]

Why is carbon rod used instead of a copper rod for a half cell containing CuCl2 solution (galvanic)


Thanks

Because copper metal is found lower on the electrochemical series than Cl2 or H2O. If we have copper there, it will be the stronger reductant, and thus will be preferentially oxidised in the reaction. In order to avoid this, a carbon rod is used which is an inert electrode that will still be able to carry out its function as the site of oxidation.

[Valyria]

Hello,

For question 15, I'm not too sure why the answer is (A). As the cell is 'generating electricity' isn't chemical energy -> electrical energy (discharging process). Thus, as the metal alloy acts as the negative electrode, the 2nd equation they've given us should represent the oxidation reaction where Ni(OH)2(s) + OH-(aq)->NiO(OH)(s) + H2O(l) + e-?

Could someone please point out any faults inherent within my explanation.

Thanks (& I've attached the photo this time)

[lzxnl]

Why is carbon rod used instead of a copper rod for a half cell containing CuCl2 solution (galvanic)


Thanks

Because copper metal is found lower on the electrochemical series than Cl2 or H2O. If we have copper there, it will be the stronger reductant, and thus will be preferentially oxidised in the reaction. In order to avoid this, a carbon rod is used which is an inert electrode that will still be able to carry out its function as the site of oxidation.

Erm what!? It depends on the other half cell as well. You're not going to be able to oxidise Cl- to Cl2 anyway because Cl- is a weaker reductant than water.

Hello,

For question 15, I'm not too sure why the answer is (A). As the cell is 'generating electricity' isn't chemical energy -> electrical energy (discharging process). Thus, as the metal alloy acts as the negative electrode, the 2nd equation they've given us should represent the oxidation reaction where Ni(OH)2(s) + OH-(aq)->NiO(OH)(s) + H2O(l) + e-?

Could someone please point out any faults inherent within my explanation.

Thanks (& I've attached the photo this time)

Metal alloy acts as negative electrode during discharge => first reaction proceeds in reverse as oxidation occurs there
Second equation proceeds as written, NiO(OH) is the oxidant as it is getting reduced. NiO(OH) isn't the metal alloy.

[Yacoubb]

Erm what!? It depends on the other half cell as well. You're not going to be able to oxidise Cl- to Cl2 anyway because Cl- is a weaker reductant than water.

Not if you have concentrated solution of Cl- ions.

[kawfee]

There is a difference between PEPTIDE LINK/BOND AND AMIDE GROUP , yeah?

VCAA 2004 Question 8 Exam 2 asks us to
Circle the peptide link

And they showed it as CONH group

WHEREAS,

I thought the peptide link is the covalent bond between C atom and N atom  ie C-N
And CONH is the amide group including a peptide link


What are your thoughts?