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Author Topic: HSC Chemistry Question Thread  (Read 1040722 times)  Share 

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Happy Physics Land

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Re: Chemistry Question Thread
« Reply #165 on: March 18, 2016, 06:13:46 pm »
+1
I have asked my teacher about moderately weak/strong acids/base producing moderately conjugates and she told me that I would not come across those except the strong and weak acids/base. Is this true? She also told me to assume CH3COOH is a weak acid and produces strong conjugate in the exam.

Hey Katherine:

If you have a look at past hsc problems, they are usually pretty straight forward. In 2009 multiple choice question 7, there was a question on the conjugate base of HSO4-. In 2014 question 10 there was a question on identifying the conjugate pair. So generally there wont be a lot of questions on solely conjugate pairs, but they usually come with buffers in our systems. For me I would just prepare for everything and yes you should write that CH3COOH produces strong conjugate in the exam but say that its strength is relative to water
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Re: Chemistry Question Thread
« Reply #166 on: March 18, 2016, 08:10:29 pm »
0
Hi, how does shaking a can of soda release CO2 in relation to the equilibrium concept?

sire123

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Re: Chemistry Question Thread
« Reply #167 on: March 18, 2016, 10:00:41 pm »
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Hey could anyone explain q23 of 2013 chem hsc paper?
23a) is what Im after, cheers.

RuiAce

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Re: Chemistry Question Thread
« Reply #168 on: March 18, 2016, 10:05:03 pm »
0
I have asked my teacher about moderately weak/strong acids/base producing moderately conjugates and she told me that I would not come across those except the strong and weak acids/base. Is this true? She also told me to assume CH3COOH is a weak acid and produces strong conjugate in the exam.
If your teacher said to assume that then even if it's incorrect go with what she said. Because that is what will get you marks in the exam

RuiAce

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Re: Chemistry Question Thread
« Reply #169 on: March 18, 2016, 10:11:58 pm »
+1
Hi, how does shaking a can of soda release CO2 in relation to the equilibrium concept?

Consider the reaction CO2(g) + H2O(l) ⇌ H2CO3(aq)   ∆H = -'ve

As we know (from being taught), the dissolution of carbon dioxide is indeed exothermic. That is, the more carbon dioxide dissolves, the more heat gets released.

When we are shaking a can of soda, we are providing the entire system with kinetic energy. As you would expect, since this is a closed environment some of this kinetic energy is transformed into heat energy. What is heat energy? Heat.

As we introduce heat into the system, according to Le Chatelier's principle the system will readjust it's equilibrium to counteract the disturbance in the system. In this case, as heat is released, when more heat is present the reverse reaction will be favoured. Effectively, this produces a greater concentration of CO2

RuiAce

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Re: Chemistry Question Thread
« Reply #170 on: March 18, 2016, 10:25:26 pm »
+1
Hey could anyone explain q23 of 2013 chem hsc paper?
23a) is what Im after, cheers.

Please copy and paste the actual question accompanying the reference to the HSC next time. :)

Our data sheet positions the reduction of silver at a much lower point than the reduction of lead. From this, we can infer that lead is indeed a more reactive metal than silver. Thus, we can anticipate that a metal displacement reaction will happen, where lead takes the place of silver. The equation can be determined easily (keeping in mind that the typical valence for lead is 2+) as follows:

Pb(s) + 2 AgNO3(aq) -> Pb(NO3)2(aq) + 2 Ag(s)

Initial moles of lead:
n(Pb) = m/M = 20.72/207.2 = 0.1 mol

Initial moles of silver nitrate:
n(AgNO3) = C*V = 0.1 * 1 = 0.1 mol

Observe the mole ratio:
1 mol of Pb reacts with 2 mol of AgNO3.
Obviously we don't have 2 mol of AgNO3 here, we only have 0.1. Let's fix up our mole ratio a bit then.

0.05 mol of Pb reacts with 0.1 mol of AgNO3.

This looks better.

Now, firstly observe that after 0.05 mol of Pb reacts, we still have a remaining 0.05 mol. Hence, n(Pb) = 0.0500 mol. That's another part of the question done.
(Take note that because the minimum amount of significant figures the question gives is 4, we must answer with 4 s.f.)
Then, observe immediately after that all of our moles of AgNO3 must've reacted (this is obviously not an equilibrium reaction, so everything reacts). Hence, n(AgNO3) = 0 mol
Obviously, 1 mol of Ag+ is present in 1 mol of AgNO3 as silver nitrate is only composed of one silver ion!
Thus, n(Ag+) = 0 mol

Now, the yield for 1 mol of Pb and 2 mol of AgNO3 is
1 mol of Pb(NO3)2, and 2 mol of Ag

This just means:
The yield for 0.05 mol of Pb and 0.1 mol of AgNO3 is
0.05 mol of Pb(NO3)2, and 0.1 mol of Ag

This directly tells us that the moles of Ag we have is:
n(Ag) = 0.100 mol

But observe that 1 mol of Pb(NO3)2 yields:
1 mol of Pb2+, and
2 mol of NO3-

Hence, n(Pb2+) = 0.0500 mol
and, n(NO3-) = 0.100 mol
« Last Edit: March 18, 2016, 10:28:40 pm by RuiAce »

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Re: Chemistry Question Thread
« Reply #171 on: March 18, 2016, 10:39:40 pm »
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Orrite ill upload the question next time as well! Thanks Rui, but are NO3- ions the spectator ions?
And would this be called a neutral species equation?

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Re: Chemistry Question Thread
« Reply #172 on: March 19, 2016, 01:08:01 am »
0
Hello  :)
I was just wondering whether I could please get some advice on how to structure and approach this question: "Assess current developments in the use of biopolymers (5 marks)". I'm not sure whether I'm meant to just refer to one biopolymer and its use or kind of just a general idea about it all and provide examples? Thanks!!

RuiAce

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Re: Chemistry Question Thread
« Reply #173 on: March 19, 2016, 06:31:27 am »
0
Orrite ill upload the question next time as well! Thanks Rui, but are NO3- ions the spectator ions?
And would this be called a neutral species equation?

Certainly. To verify that, take note that the initial moles and final moles of the nitrate ions are both 0.100 mol.

And no. A neutral species equation is one which does not involve any changes in the charge. Note that this would actually be a redox reaction, where lead is being oxidised and silver ions are being reduced.

RuiAce

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Re: Chemistry Question Thread
« Reply #174 on: March 19, 2016, 06:33:16 am »
+1
Hello  :)
I was just wondering whether I could please get some advice on how to structure and approach this question: "Assess current developments in the use of biopolymers (5 marks)". I'm not sure whether I'm meant to just refer to one biopolymer and its use or kind of just a general idea about it all and provide examples? Thanks!!

There are factors about biopolymers (e.g. renewability, possible feasible properties) that hold true for all biopolymers. However, in the scope of the chemistry course you will only be asked to make specific reference to one. It may also help to contrast to traditional petroleum-derived polymers such as the obvious polyethylene.

The two most common ones I've found in the HSC are polyhydroxybutyrate and polylactic acid

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Re: Chemistry Question Thread
« Reply #175 on: March 19, 2016, 01:03:15 pm »
0
And also, I have another q:
A chemist analysed aspirin tablets for quality control. The initial step of the analysis was the standardisation of a NaOH solution. Three 25.00 mL samples of a 0.1034 mol L–1 solution of standardised HCl were titrated with the NaOH solution.The average volume required for neutralisation was 25.75 mL.
(a) Calculate the molarity of the NaOH solution.
...
...
...
...
Three flasks were prepared each containing a mixture of 25 mL of water and 10 mL
of ethanol. An aspirin tablet was dissolved in each flask. The aspirin in each solution
was titrated with the standardised NaOH solution according to the following equation:
C9H8O4(aq) + NaOH(aq) → C9H7O4Na(aq) + H O(l) 2
The following titration results were obtained.
Tablet Volume (mL)
1 16.60
2 16.50
3 16.55
(b) (i) Calculate the average mass (mg) of aspirin per tablet.
...
...
...
...
...
...
...
(ii) Why was it necessary to include the ethanol in the mixture?
...
...
« Last Edit: March 19, 2016, 01:08:03 pm by sire123 »

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Re: Chemistry Question Thread
« Reply #176 on: March 19, 2016, 06:20:51 pm »
0
Heyyyaaa there!! I have a Question that has been bugging me.. would love to get your advice / help with it! :))

Q: Why would the neutralisation of HCl and Na2CO3 require the addition of methyl orange indicator? I thought the best suited indicator would be bromothymol blue because this titration involved a strong base and a strong acid - hence a neutral salt? (or is Na2CO3 a weak base)?

Got me reeaaallll caught up. Thanks in advance!  8)

RuiAce

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Re: Chemistry Question Thread
« Reply #177 on: March 19, 2016, 06:43:24 pm »
+2
And also, I have another q:
A chemist analysed aspirin tablets for quality control. The initial step of the analysis was the standardisation of a NaOH solution. Three 25.00 mL samples of a 0.1034 mol L–1 solution of standardised HCl were titrated with the NaOH solution.The average volume required for neutralisation was 25.75 mL.
(a) Calculate the molarity of the NaOH solution.
...
...
...
...
Three flasks were prepared each containing a mixture of 25 mL of water and 10 mL
of ethanol. An aspirin tablet was dissolved in each flask. The aspirin in each solution
was titrated with the standardised NaOH solution according to the following equation:
C9H8O4(aq) + NaOH(aq) → C9H7O4Na(aq) + H O(l) 2
The following titration results were obtained.
Tablet Volume (mL)
1 16.60
2 16.50
3 16.55
(b) (i) Calculate the average mass (mg) of aspirin per tablet.
...
...
...
...
...
...
...
(ii) Why was it necessary to include the ethanol in the mixture?
...
...

a)
NaOH(aq) + HCl(aq) -> H2O(l) + NaCl(aq)
The reaction immediately shows that the mole ratio between sodium hydroxide and hydrochloric acid is one to one.
n(NaOH) = n(HCl)
Using n=CV
[NaOH] * V(NaOH) = [HCl] * V(HCl)
[NaOH] * 25.75mL = 25.00mL * 0.1034 mol L-1
[NaOH] = 0.100388... mol L-1
Note that for this part of the question you would round to 4 significant figures and give your answer as 1.004*10-1 mol L-1 but for the remainder of the question you use the UNROUNDED answer in your calculator

b) (i) This can obviously be done by inspection, but as the HSC requires proper calculations the average is obviously
V(NaOH) = (16.60+16.50+16.55)mL/3 = 0.01655L
Important aside: It's not that easy to copy and paste a table. In the actual HSC question, we assume that the tablet is the same, but rather human error caused the differences in the calculated volumes for NaOH that it was titrated against. So the average volume is the volume of NaOH used for titration.

Observe that the mole ratio is once again one-to-one. This means
n(C9H8O4)
= n(NaOH)
= [NaOH] * V(NaOH)
= answer from part a) * 0.01655
= 1.6614278...*10-3 mol

Using the fact that m = n*MM
m = 1.6614278...*10-3 * 180.154
= 0.2993g (correct to 4 s. f. here)

b) (ii) The question was generous here to make this a one mark question. And so they should, because they didn't give us a diagram of what aspirin looks like on the molecular level.

So the only logical answer to using ethanol in the mixture is to promote the solubility of aspirin (that's the answer). Just by looking at C9H8O4 we can make a few assumptions:
a) There's quite a fair few carbons. Expect some kind of a chain, or benzene ring (tbh, yes benzene ring in the actual molecule) which ethanol will easily dissolve.
b) The 4 oxygens could all form -OH groups for all we know but obviously we don't know. Even then, both water AND ethanol interact through hydrogen bonding and dipole-dipole interactions when hydroxyl groups are present, so you can still safely assume that ethanol increases solubility.

But basically, it's cause there's obviously going to be carbon chains, which essentially makes ethanol a more effective solvent.
Heyyyaaa there!! I have a Question that has been bugging me.. would love to get your advice / help with it! :))

Q: Why would the neutralisation of HCl and Na2CO3 require the addition of methyl orange indicator? I thought the best suited indicator would be bromothymol blue because this titration involved a strong base and a strong acid - hence a neutral salt? (or is Na2CO3 a weak base)?

Got me reeaaallll caught up. Thanks in advance!  8)

Na2CO3 is of course, a weak base. The only strong bases in the course are essentially NaOH, KOH and all the other group 1 hydroxides.
« Last Edit: March 19, 2016, 06:50:53 pm by RuiAce »

Happy Physics Land

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Re: Chemistry Question Thread
« Reply #178 on: March 19, 2016, 07:16:15 pm »
+1
Hello  :)
I was just wondering whether I could please get some advice on how to structure and approach this question: "Assess current developments in the use of biopolymers (5 marks)". I'm not sure whether I'm meant to just refer to one biopolymer and its use or kind of just a general idea about it all and provide examples? Thanks!!

Hey liiz:

The way I would first approach this question is to highlight key words, its really useful because it tells you what to focus on. Assess tells us that we are making a judgment, current developments tells us we need to perhaps provide some statistics or relate the emergence of biopolymers to what is happening in the world right now. Use of biopolymers should tell us that we should provide two examples of uses of biopolymers (relate it to the polymer's properties). A lot of people would start to talk about how a specific biopolymer is made, and explain the complicated chemistry behind the production process and its properties. These descriptions however wouldnt reward you any marks because our focus is "current development" and "uses of biopolymer".

Personally, this is how I would structure this response, but of course there are many approaches that you can adopt.

1. Evaluate the current development (i.e. something like "use of biopolymers in commodities have undergone significant growth over the recent years (insert a statistic if possible))
2. Provide examples of uses of biopolymers (2 examples, include specific examples if possible, e.g. because of its rigid property, PLA has been extensively used in Danone yoghurt cups)
3. Describe the reasons in the current world which have encouraged the development of biopolymers (e.g. the depletion of the non-renewable petroleum have resulted in the rising in price of fossil fuels, making the production of biopolymers increasingly economical)
4. List 3 advantages of biopolymers in general
5. Assess the limitations of biopolymers (e.g. despite these positive aspects about biopolymers, companies are currently experiencing some difficulties with manufacturing them in large amounts and the process of production biopolymers is still not as efficient as using petrochemicals.)
6. Restate your assessment of the development of biopolymers (Judgment: There has a been a rapid and significant current development of biopolymers. I know this sounds a bit clumsy, but by writing this, the teacher knows you are achieving the "assess" part of the question)

Best Regards
Happy Physics Land
« Last Edit: March 19, 2016, 07:27:13 pm by Happy Physics Land »
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Happy Physics Land

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Re: Chemistry Question Thread
« Reply #179 on: March 19, 2016, 07:25:30 pm »
+2
Heyyyaaa there!! I have a Question that has been bugging me.. would love to get your advice / help with it! :))

Q: Why would the neutralisation of HCl and Na2CO3 require the addition of methyl orange indicator? I thought the best suited indicator would be bromothymol blue because this titration involved a strong base and a strong acid - hence a neutral salt? (or is Na2CO3 a weak base)?

Got me reeaaallll caught up. Thanks in advance!  8)

Hey missrissole!

Completely agree with what Rui said, just want to drop in my two pennies here. Na2CO3 definitely is a weak base, and like what Rui said, majority of the strong bases come from hydroxides of group 1 alkali metals and group 2 alkaline earth metals. Since HCl is a strong acid, the pH of reaction mixture at equivalence point would be below 7 (i.e. acidic). Hence methyl orange would be the best indicator here because it is suitable for use in the lower acidic range. Bromothymol blue is useful indicator for strong acid/strong base titration because the equivalence point of the reaction has a pH of 7. Bromothymol blue displays a green colour for pH range 6.0 - 7.6, hence it would be useful for evaluating neutral equivalence points.

Hope my answer helps! If you have further questions, please dont be hesitant to ask! :D

Best Regards
Happy Physics Land
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