HSC Chemistry 2021 Exam Solutions (1 Viewer)

CM_Tutor

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My answer to Q36 would be:

H2SO3 (aq) + 2 H2O (l) <---in equilibrium---> 2 H3O+ (aq) + SO32- (aq)


Even giving a mark for appropriate significant figures (which is 2, because the p''K''a values are given to 2 decimal places), I struggle to give 5 marks for this without being overly generous, but I guess it could be:
  • 1 mark for the expression for K
  • 1 mark for rearranging to show the component Ka expressions​
  • 1 mark for substituting the Ka values from 10-pKa
  • 1 mark for the numerical answer (ECD from expression given)​
  • 1 mark for answering to 2 sig. fig.​
 

Life'sHard

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My answer to Q36 would be:

H2SO3 (aq) + 2 H2O (l) <---in equilibrium---> 2 H3O+ (aq) + SO32- (aq)


Even giving a mark for appropriate significant figures (which is 2, because the p''K''a values are given to 2 decimal places), I struggle to give 5 marks for this without being overly generous, but I guess it could be:
  • 1 mark for the expression for K
  • 1 mark for rearranging to show the component Ka expressions​
  • 1 mark for substituting the Ka values from 10-pKa
  • 1 mark for the numerical answer (ECD from expression given)​
  • 1 mark for answering to 2 sig. fig.​
I can’t for the life of me remember if I left my answer as 10^-8.99 or 1.0x10^-9. If I did leave it as 10^-8.99 do you think this is sufficient as an answer? It still has 2 sig figs.
 
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CM_Tutor

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I can’t for the life of me remember if I left my answer as 10^-8.99 or 1.0x10^-9. If I did leave it as 10^-8.99 do you think this is sufficient as an answer? It still has 2 sig figs.
The wording was "calculate the equilibrium constant" so I think they will expect an answer in scientific notation.

Further, 10-8.99 is (as a number) an exact value and my answer is a 2sf approximation to it. I am not inclined, on first thinking, to call 10-8.99 an answer to 2sf, though I might in a case like 10-8.990347... = 10-8.99 (2sf)... though even then, what if I said 10-8.99 = 10-9+0.01 = 100.01 x 10-9 and claimed this is 1 sf?

Given the data, I don't think that rounding the answer to K = 10-9 is justified, either.
 

jazz519

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I can’t for the life of me remember if I left my answer as 10^-8.99 or 1.0x10^-9. If I did leave it as 10^-8.99 do you think this is sufficient as an answer? It still has 2 sig figs.
Not sure if this is correct but back when I did HSC, my tutor was a HSC marker and she said something about only 1 question in the exam being chosen for significant figure marking. So if it's that question they select which may be the case if it's 5 marks then you will lose a mark. If they don't choose that you can probably get away with it
 

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