Ionisation (1 Viewer)

[kimmeh]

Can someone explain to me why C is the correct answer?

 

[kimmeh]

i've already gotten the answer as D from a uni student, and thats what i though originally. any comments?

 

[Xayma]

It is D. Unless z is a polyatomic acid.

 

[kimmeh]

what polyatomic? where'd that come from?

 

[Xayma]

More then one Hydrogen is able to be ionised.

For example Sulfuric Acid only has about 70% ionisiation since HSO₄^-1 doesnt fully ionise but it will still have a lower pH then an equal concentration of HCl (that is sulfuric acide not hydrogen sulfate)

 

[04er]

I agree with Xayma

 

[04er]

they can't expect us to assume the acid is polyatomic, that would need to be given. Kimmeh, where did you get this question from?

 

[kimmeh]

its from 2001 CSSA Trial Paper

but even if it was polyatomic, theres no indication whatsoever that it is. it isnt even in our syllabus is it?

 

[CM_Tutor]

The answer is D. There was a typo in the answers to the 2001 CSSA trial over the answer to this question - corrected on most versions you see, but not all.

The notion of 'polyatomic' acids was discussed above. I think the term being sought is polyprotic acids, ie acids that can undergo more than one ionisation, let sulfuric acid. Note that the answer would be D irrespective of polyprotism, and polyprotism is in the syllabus.

 

[Ragerunner]

The CSSA are very good at giving really bad answers in their answer sheet.

In the physics 2003 CSSA paper...out of 15 multiple choice questions, 6 of them were wrong.

Absolutely shocking...

and their answers for the high mark questions aren't exactly any good either. I'd suggest not to just use their answers and assume what they write for a 6 mark question is actually worth 6 marks.

 

[CM_Tutor]

Originally posted by Ragerunner
I'd suggest not to just use their answers and assume what they write for a 6 mark question is actually worth 6 marks.

I think this is a very good piece of advice.

 

[:: ck ::]

More then one Hydrogen is able to be ionised.

isnt dat polyprotic?

edit : bleh didn c wut CM posted

 

[Xayma]

Originally posted by CM_Tutor
The notion of 'polyatomic' acids was discussed above. I think the term being sought is polyprotic acids, ie acids that can undergo more than one ionisation, let sulfuric acid. Note that the answer would be D irrespective of polyprotism, and polyprotism is in the syllabus.

Oh yeah polyatomic means multiple atoms, so sulfuric acid would have a higher degree of ionisation then hydrochloric

 

[CM_Tutor]

Originally posted by Xayma
Oh yeah polyatomic means multiple atoms, so sulfuric acid would have a higher degree of ionisation then hydrochloric

Polyatomic = more than one atom - ie the ion SO₄^2- is polyatomic

This is not linked to polyprotism. Hydrochloric acid is monoprotic, but it has an overall higher degree of ionisation than does diprotic sulfuric acid as the second ionisation of sulfuric acid does not complete, as you pointed out above, but hydrochloric acid's one and only ionisation does complete. Now, the sulfuric acid would have a lower pH than would the hydrochloric acid (for solutions of equal concentration) due to its polyprotism, but that isn't the same thing.

 

[Xayma]

So if a question was like:

Acid...Concentration...pH

X...0.1M...2
Y...0.1M...1.8

Y is a diprotic acid whereas X is a monoprotic acid.

Then X would have a greater degree of ionisation?

 

[CM_Tutor]

Assuming that you meant 'acid' and not 'element', Y would definitely be stronger than X, and this is the question you would be asked.

Essentially, you raise a good point, but the problem lies in the use of an imprecise term like 'degree of ionisation'. A problem like this needs a mathematical treatment if you want to go beyond the qualitative X is weaker than Y result - unfortunately, the basis for the calculations is outside the syllabus.

 

[Xayma]

Oh well thats what uni chemistry is for, and now I just need to get out of this bad habit of writing down the wrong words (I know what I mean but I need to study more to make sure I write it down correctly)