HSC Chemistry 2017

[GodlyOne]

For Q18, why does having the same rate of reaction necessarily mean they have same molar ratio, wouldn't it be C because on the graph Y had a higher concentration than Z indicating that per 1 molecule of Y, 3 molecules of Z were needed to react with Y to form X?

[browniesandpuppies]

Any rough idea what the raw mark cut off would be this year for band 6?

[jiaruilu]

Um pretty sure question 3 is A. Alphabetical order!

[cindy_smilez]

For 26a (2 effects of SO2), is it accepted if you only write acid rain but describe 2 effects of acid rain??

[tripat23]

exactly what I wrote !

[Merin Thomas]

reckon 80 would be the cut off? or around 82? ~~~

[jenna20354]

"The pH level of the resulting salt solution is not 7, but was instead slightly below 7 because we are reacting a strong base with a weak acid. The final product NaCH3COO(aq) --> Na+(aq) + CH3COO-(aq). CH3COO-(aq) is a weak base, therefore the endpoint is slightly basic."

When you say "but was instead slightly below seven" did you mean "but instead was slightly above seven?" as bases have a pH higher then 7?

[lakwas7]

"The pH level of the resulting salt solution is not 7, but was instead slightly below 7 because we are reacting a strong base with a weak acid. The final product NaCH3COO(aq) --> Na+(aq) + CH3COO-(aq). CH3COO-(aq) is a weak base, therefore the endpoint is slightly basic."

When you say "but was instead slightly below seven" did you mean "but instead was slightly above seven?" as bases have a pH higher then 7?

I believe he meant to say pH is slightly higher than 7 but he had to rush the questions since he was at uni

Also did anyone get a pH of 1.6 for Q20. I looked at solutions for a similar question and it is similar to my working. Instead of multiplying two, I would have thought since they react in a 2:1 ratio you divide it by two in order to find the excess moles. If anyone could pinpoint what I'm missing out on that'd be great

[Invat]

I believe he meant to say pH is slightly higher than 7 but he had to rush the questions since he was at uni

Also did anyone get a pH of 1.6 for Q20. I looked at solutions for a similar question and it is similar to my working. Instead of multiplying two, I would have thought since they react in a 2:1 ratio you divide it by two in order to find the excess moles. If anyone could pinpoint what I'm missing out on that'd be great

A lot people got 1.6, i think there is a mistake with the question

[Invat]

Also i think you may have missed the second part of question 28 which i believe was a ethanol combustion question.

[Nick Seb]

Also i think you may have missed the second part of question 28 which i believe was a ethanol combustion question.

Yeah I was looking for that one too, I think I had 11.3*1360kJ or so, something like 15500kJ per kg of CO2 produced.

~Nick

[Nick Seb]

I believe he meant to say pH is slightly higher than 7 but he had to rush the questions since he was at uni

Also did anyone get a pH of 1.6 for Q20. I looked at solutions for a similar question and it is similar to my working. Instead of multiplying two, I would have thought since they react in a 2:1 ratio you divide it by two in order to find the excess moles. If anyone could pinpoint what I'm missing out on that'd be great

The 2:1 ratio only applies to the amount of barium hydroxide reacting. 0.0004moles of Ba(OH)2 requires twice the moles of HCl to completely react, i.e. 0.0008 moles. Thus, you subtract 0.0008 from the initial 0.002 (because that's what reacted and got neutralised), leaving 0.0012moles. Find concentration, find pH, which gave the same answer as OP.
Hope this helped,
~Nick

[Happy Physics Land]

I believe he meant to say pH is slightly higher than 7 but he had to rush the questions since he was at uni

Also did anyone get a pH of 1.6 for Q20. I looked at solutions for a similar question and it is similar to my working. Instead of multiplying two, I would have thought since they react in a 2:1 ratio you divide it by two in order to find the excess moles. If anyone could pinpoint what I'm missing out on that'd be great

Thank you for understanding! With Q20, because of the 2:1 ratio between HCl and the Ba(OH)2, you need to minus twice the Ba(OH)2 from moles of HCl (as Ba(OH)2 is the limiting reagent!)

[Happy Physics Land]

"The pH level of the resulting salt solution is not 7, but was instead slightly below 7 because we are reacting a strong base with a weak acid. The final product NaCH3COO(aq) --> Na+(aq) + CH3COO-(aq). CH3COO-(aq) is a weak base, therefore the endpoint is slightly basic."

When you say "but was instead slightly below seven" did you mean "but instead was slightly above seven?" as bases have a pH higher then 7?

Yes thats exactly what I meant! Thank you for pointing that out!

[angelahchan]

for q28 for the advantages/disadvantages of ethanol is it acceptable to talk about more general disadvantages e.g. opportunity cost of land to grow crops which are instead used to make fuel, or did it specifically have to link to its function in cars?