HSC Chemistry Question Thread

[Mate2425]

Hey guys, I was wondering how you answer HSC 2017 Chemistry Multiple Choice Q20. http://educationstandards.nsw.edu.au/wps/wcm/connect/cb65ca25-fc28-4e21-8f2d-4886bf176ddd/2017-hsc-chemistry.pdf?MOD=AJPERES
I had a look at Matrix answers on their website and am not sure where they plucked their numbers from.
Thank you!

Also Q24b HSC 2015 please.... so confused thank you https://www.boardofstudies.nsw.edu.au/hsc_exams/2015/guides/2015-hsc-mg-chemistry.pdf

[jazcstuart]

Hey guys, I was wondering how you answer HSC 2017 Chemistry Multiple Choice Q20. http://educationstandards.nsw.edu.au/wps/wcm/connect/cb65ca25-fc28-4e21-8f2d-4886bf176ddd/2017-hsc-chemistry.pdf?MOD=AJPERES
I had a look at Matrix answers on their website and am not sure where they plucked their numbers from.
Thank you!

I have attached my solution, hope it makes sense.
Essentially you have to find the number of moles remaining of the substance in excess (in this case HCl) after neutralisation, then find the concentration using the volume of both solutions. You can then use this to find pH.
If you don't understand my explanations feel free to ask 

[Mate2425]

I have attached my solution, hope it makes sense.
Essentially you have to find the number of moles remaining of the substance in excess (in this case HCl) after neutralisation, then find the concentration using the volume of both solutions. You can then use this to find pH.
If you don't understand my explanations feel free to ask 

Thank you so much for your help, makes much more sense!!
If possible could you please also help me with my new edit on the 2015 Question above, i would greatly appreciate it!! 

[jazcstuart]

Thank you so much for your help, makes much more sense!!
If possible could you please also help me with my new edit on the 2015 Question above, i would greatly appreciate it!! 

Yeah sure! (I love answering questions because it helps me clarify my understanding )

So this is a buffer solution, which is a really important concept to understand. This is because acetate ions (from the sodium acetate) is the conjugate base of acetaic acid. We have to use Le Chatlier's Principle because the solution exists in equilibrium:

CH₃COOH + H₂O <---> CH₃COO^- + H₃O⁺

The OH^- ions added from the sodium hydroxide solution react with hydronium ions (H₃O⁺), reducing their concentration. Therefore according to Le Chatlier's Principle, the equilibrium shifts to the left, minimising the change in pH (as more H₃O⁺ ions are produced to counteract the addition of OH^-)
Although this question doesn't ask for it, it is similar when an acid is added. The concentration of H₃O⁺ is increased, shifting to the equilibrium to the right according to Le Chatlier's Principle, again minimising the change in pH.

[Dragomistress]

Hi,
What is the net ionic equation of 2NaCl+H2O-->Cl2+H2+2NaOH?

[jazcstuart]

Hi,
What is the net ionic equation of 2NaCl+H2O-->Cl2+H2+2NaOH?

Hi, first of all to balance the chemical equation you need a 2 infront of H2O.
Essentially when you write net ionic equations you are determining the chemical species that undergo a reaction, ie. they change oxidation state. I think you can only split substances up into their individual ions if they are in solution. H2O is a covalent molecule so it remains the same, so you should get:

2Na⁺ + 2Cl^- + 2H₂O ---> Cl₂ + H₂ + 2Na⁺ + 2OH^-

Then you remove the spectator ion (Na⁺, doesn't change oxidation state) to get:

2Cl^- + 2H₂O ---> Cl₂ + H₂ + 2OH^-   (hope this is right)

Hope this makes sense 

[Dragomistress]

Hello again!
What is the purpose of using concentrated sulfuric acid in esterification reactions?

[Dragomistress]

Also, how is the pH of the final solution 12.1?
In a reaction, 30.0mL of a 0.106mol/L NaOH is added to 25.0 mL of 0.100 mol/L HCl solution.

[Bri MT]

Hello again!
What is the purpose of using concentrated sulfuric acid in esterification reactions?

The sulfuric acid acts as a catalyst for the reaction.

if you think about the equation, it's:   alcohol + carboxylic acid ---> ester + water
but this is actually a reversible reaction! so if you have lots of water, you're going to favour the reverse reaction and therefore have less ester made.
That's why the sulfuric acid needs to be concentrated, as otherwise you're adding too much extra water

Hope this helps


(also, in future please edit the post you already have rather than posting twice in a row)

[Dragomistress]

Hello again!
I am wondering what is the structural formula for ethanol and butanoic acid?

[Bri MT]

Hello again!
I am wondering what is the structural formula for ethanol and butanoic acid?

Eth indicates that you have 2 carbons, and -ol indicates that you have a OH group. From this you get the formula C2H5OH. Everything will be connected to the carbons.
But indicates that there are 4 carbons, oic acid indicates that one of those carbons is part of a COOH (both oxygens are bonded to the carbon, one by a double bond and one by a single bond)


I've made VCE chemistry notes on this topic that might also be useful to you (search for organic nomenclature in the notes section if you want to find it)

[Dragomistress]

Oh, sorry. I forgot to write the question. How am I meant to draw the corresponding ester and what is the name of that ester?

[Bri MT]

Oh, sorry. I forgot to write the question. How am I meant to draw the corresponding ester and what is the name of that ester?

The alcohol loses its OH and the acid loses a H (which form water).
The bond connecting the two occurs where these atoms were lost.

The alcohol gets an "yl" ending (which replaces the ol) and the acid gets an "oate" ending (which replaces the oic acid). Don't put a space between the two

[Mate2425]

Hey guys for HSC 2017 Q22a. Would you marked incorrect for drawing the interpolation of part b. on the graph even though you have drawn everything else correctly. Also was wondering why answer to Q20MC is A and not answer B in the HSC 2016 Paper.
Also with HSC 2015 past paper wouldn't you be correct in saying for Q6MC that CH4 is the most suitbale replacement to CFC's as they have no c-cl and c-F bonds thus no free radicals and hence no Ozone depletion.They remark the answer as B??
Thank you.
HSC 2017: http://educationstandards.nsw.edu.au/wps/wcm/connect/cb65ca25-fc28-4e21-8f2d-4886bf176ddd/2017-hsc-chemistry.pdf?MOD=AJPERES
HSC 2016: https://educationstandards.nsw.edu.au/wps/wcm/connect/e76232cd-1d84-4d3e-92ce-ea41546810f6/2016-hsc-chemistry.pdf?MOD=AJPERES&CACHEID=ROOTWORKSPACE-e76232cd-1d84-4d3e-92ce-ea41546810f6-lHPDE.v

[blasonduo]

snip

I doubt you'd lose a mark for it as the marking feedback didn't state that better responses did not extrapolate, but a good rule of thumb is unless you know for CERTAIN that the relationship is linear all the way to infinity (generally by an equation), then don't extrapolate.

Here's an example of why extrapolating when unsure could give you very wrong results.



(This is a physics example and not a chemistry example, I know, sorry, but it conveys my point)

As you can see, when we plot the electric magnetic field magnitude with distance of a conducting sphere, it starts out very linear, but once the distance is greater than the radius of the conductor (R), it decreases exponentially. So, if you were given 5 points that all lied between 0 and R, you'd believe that it was linear, however, since we aren't CERTAIN of the equation, we shouldn't extrapolate, because in this case, we would be very wrong. This is similar to why we don't do it in the chemistry question.

As for Q20 in 2016, AAS works by comparing the same wavelength before,and after to see how much of the intensity was absorbed, you wouldn't get any proper results comparing the intensities of 2 different wavelengths, because well, they are different, and different wavelengths absorb differently.

Lastly, HFC's are the offical way for improving, and yeah you are correct, however the equation would be CH4  +  Cl2 -> CH3Cl  +  HCl. This is producing hydrochloric acid! As the question wants the one that reduces the environmental impact the most, I'd go with B.

Hope this helps!