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Thread: HSC 2016 Chemistry Marathon

  1. #76
    Ancient Orator leehuan's Avatar
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    Re: HSC Chemistry Marathon 2016

    (That sodium hydrogen carbonate is giving me implications of an actual cube of it. Because otherwise why would the limestone be powdered. Note: Yes, the question was in this regard, ambiguous)

    H(+) + OH(-) -> H2O; ΔH≈-57kJ mol^-1
    The reaction between a strong acid and a strong base is highly exothermic. This is true for all strong bases and acids interacting as the enthalpy change is determined as a result of the formation of the water molecule in neutralisation. Therefore, the enthalpy change for a strong acid-base neutralisation is always approximately -57kJ mol^-1. However, when used with a weak base, this can be reduced to -47kJ mol^-1.

    This makes the 2M NaOH solution immediately inviable for use, as the heat released from the reaction will be unnecessarily large. Additionally, if by accident we use an excess amount of NaOH to neutralise the HCl, we may only cause another bad situation where we need to neutralise a base instead. This is equally problematic as whilst 10M HCl poses acid burn hazards, 2M NaOH will create risks of base irritation.

    Both NaHCO3 and CaCO3 are weak bases, and thus have both the advantages of causing limited irritation if using excess, as well as reducing the heat of reaction.

    The NaHCO3, being a weaker base than powdered limestone (and of course, NaOH) is arguably a better choice than spreading the limestone around due to the heat released from the reaction being potentially minimised further. However, it is unsafe to essentially roll a cube of a substance around simply for the process of neutralisation. Whilst the process of neutralisation should be slow to minimise the rate of release of heat, it is quite inconvenient to have to carry the cube around due to it's small surface area (A 2L spill will be massive) and spread it out to allow the neutralisation process to take place. The user is also essentially exposed to prolonged heat exposure, which may cause burns in itself.

    These problems do not outweigh the situation with heat released, thus the most viable option here is therefore the powdered CaCO3 as we will be able to spread it out evenly, then leave the laboratory as soon as possible to minimise contact with the substances. This neutralisation process will also allow for a more safer alternative to clean up later.

    (Note: A better solution is to soak up the acid in sand first, then neutralise the acid elsewhere)

    Quote Originally Posted by leehuan View Post
    NEXT QUESTION:

    A student sets up an enclosed environment to simulate acid rain. The system initially consists of only sulfur dioxide gas and litmus paper. When he sprayed water into this environment, the litmus turned red rapidly.

    a) Use relevant equations to interpret the results of the experiment and assess its validity. (4)
    b) Evaluate the impacts of increasing levels of sulfur dioxide in the environment today. (4)
    Shuuya and chanandlerbong like this.

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    Re: HSC Chemistry Marathon 2016

    Quote Originally Posted by RachelGreen View Post
    Petroleum is a non-renewable fossil fuel, and supplies are being rapidly depleted due to heavy demands for petrochemicals for a variety of uses such as fuels, plastics, lubricants, solvents etc. The demand is increasing as world's populations increases. Consequently, crude oil is being rapidly depleted due to HIGH demand and limited crude oil reserves. Furthermore, the need for alternative sources of petrochemical products derivatives come down to two significant points: environmental impact and scarcity.

    - Majority of crude oil is used up as a fuel, the consumption of fuel products is detrimental upon the environment which releases pollutants in the atmosphere (octane - major component of petrol, burns incompletely). In comparison to other potential fuels such as ethanol, the current petrol products burns relatively uncleanly, leading to environmental problems.

    - Most polymers are currently derived from petroleum are currently non-biodegradable which places a considerable strain on our landfills. Alternative sources esp. biopolymers, are biodegrable and would alleviate such problems

    - Another reason why alternative sources are needed because petrochemical products are derived from non-renewable sources of crude oil, with chemists placing the lifespan on current petroleum supplies well under 50 years, alternative sources are required simply due unsustainable trends

    Alternatives sources such as biomass may be used, however they are more expensive than crude oil. In addition, using ethanol as an example for alternative sources; new infrastructure needs to be constructed (such as fermentation plants for ethanol), a process which takes time and money and just isn't economically viable currently.

    84% of crude oil is used to produce energy. This includes petrol and diesel for cars. Potential alternative sources too alleviate these problems would be ethanol where it will meet with not only future energy needs, but with material needs as well. Ethanol can be produced by fermenting sugars from sugar cane crops:
    C6H12O6 (aq) ----(yeast enzymes) ---> 2C2H5OH (aq) + 2CO2(g)

    Benefits:
    - Ethanol is able to be used as a petrol supplement, because it undergoes combustion: C2H5OH (l) + 3O2 (g) -> 2CO2 (g) + 3H2O (l) + heat
    It can be used as a substitute because it is a renewable resource as it is manufactured from carbohydrates such as glucose and starch that are produced by photosynthesis by plants. The products of its combustion (CO2 and H2O) are the reacts needed by plants for photosynthesis: 6CO2 (g) +
    6H2O (l) ----light----> C6H12O6 (aq) + 6O2 (g)

    - Burns completely in oxygen (cleaner burning fuel) in contrast to other fuels that undergoes incomplete combustion thus producing toxic CO and carcinogenic soot

    Ethanol combined with its high energy-per-mole output, cleaner burning nature and ease of transport, ethanol has great potential as an alternative fuel.

    Please correct my response!

    NEXT QUESTION:
    Outline two methods of producing ethanol. In your answer, construct chemical equations to demonstrate ONE renewable and ONE non-renewable method of ethanol production. (4 marks)

    Hey RachelGreen did you write all these answers of the top of your head?
    How do you remember all this?
    Makes me feel dumb!!!

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    Ancient Orator leehuan's Avatar
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    Re: HSC Chemistry Marathon 2016

    Quote Originally Posted by ragingcurry View Post
    Hey RachelGreen did you write all these answers of the top of your head?
    How do you remember all this?
    Makes me feel dumb!!!
    It's always been revising over notes (printed + HIGHLIGHTING for truly difficult stuff for me to remember), and practice questions.

    Because there were certain dot points that I just need reaffirming what I knew every now and then (e.g. industry and medicine radioisotope), then those that I had a fair grasp on but I'd always lose info on x and y (potentials of polyethylene), then stuff I really struggled on (for me one of them was actually just the reflux practical).

    If you just note take in class without trying to think about what's what, that doesn't help either. If you do computer notes, that's also a bit damaging because writing is one of the best forms of study. That's why if you want to properly know your stuff, copy out your notes over and over again, and do past papers.
    __________________________________________________ _

    Bump

    A student sets up an enclosed environment to simulate acid rain. The system initially consists of only sulfur dioxide gas and litmus paper. When he sprayed water into this environment, the litmus turned red rapidly.

    a) Use relevant equations to interpret the results of the experiment and assess its validity. (4)
    b) Evaluate the impacts of increasing levels of sulfur dioxide in the environment today. (4)

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    Re: HSC Chemistry Marathon 2016

    Quote Originally Posted by ragingcurry View Post
    Hey RachelGreen did you write all these answers of the top of your head?
    How do you remember all this?
    Makes me feel dumb!!!

    Tbh, I just study a lot. If you're having problems answering questions here, then you should use UR notes to help you answer

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    Re: HSC Chemistry Marathon 2016

    Determine the number of moles of chromium atoms in 140.0g chromium(III) oxide.




    I thought I knew how to do this but I can't get the answer given in the solutions.
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    Re: HSC Chemistry Marathon 2016

    Quote Originally Posted by Flop21 View Post
    Determine the number of moles of chromium atoms in 140.0g chromium(III) oxide.




    I thought I knew how to do this but I can't get the answer given in the solutions.
    (Relevant page on Wikipedia: https://en.wikipedia.org/wiki/Chromium(III)_oxide.)

    From the Wikipedia link, the molar mass of chromium (III) oxide is 151.9904 g/mol. (You could also have found this from a Periodic Table if you knew that chromium (III) oxide is Cr2O3.)

    I'm lazy, so let M = 151.9904 g/mol (the molar mass) and let m = 140.0 g (the mass of the sample).

    Hence the moles of chromium (III) oxide in the sample n = m/M.

    Hence the number of chromium atoms in the sample is 2n = 2m/M, as there are 2 Cr's per Cr2O3. The rest is a calculator exercise.

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    Re: HSC Chemistry Marathon 2016

    Quote Originally Posted by InteGrand View Post
    (Relevant page on Wikipedia: https://en.wikipedia.org/wiki/Chromium(III)_oxide.)

    From the Wikipedia link, the molar mass of chromium (III) oxide is 151.9904 g/mol. (You could also have found this from a Periodic Table if you knew that chromium (III) oxide is Cr2O3.)

    I'm lazy, so let M = 151.9904 g/mol (the molar mass) and let m = 140.0 g (the mass of the sample).

    Hence the moles of chromium (III) oxide in the sample n = m/M.

    Hence the number of chromium atoms in the sample is 2n = 2m/M, as there are 2 Cr's per Cr2O3. The rest is a calculator exercise.
    Oh I was looking at the formula for chromuium(III) oxide and saw there was 2 chromium atoms so molar mass of the chromium atoms are 2* 51.996(or 52) then calculating from there. Is this not correct?

    Also calculating 1511.9904/140 = 0.92, but answers say 1.842?
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    Re: HSC Chemistry Marathon 2016

    Quote Originally Posted by Flop21 View Post
    Oh I was looking at the formula for chromuium(III) oxide and saw there was 2 chromium atoms so molar mass of the chromium atoms are 2* 51.996(or 52) then calculating from there. Is this not correct?

    Also calculating 1511.9904/140 = 0.92, but answers say 1.842?
    The answer is 2*m/M = 2*140/(151.9904) (explanation given in my previous post).

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    Re: HSC Chemistry Marathon 2016

    Quote Originally Posted by Flop21 View Post
    Oh I was looking at the formula for chromuium(III) oxide and saw there was 2 chromium atoms so molar mass of the chromium atoms are 2* 51.996(or 52) then calculating from there. Is this not correct?

    Also calculating 1511.9904/140 = 0.92, but answers say 1.842?
    What was the method you were using before? I think you were treating molar mass as M' = 2m_Cr, where m_Cr is the molar mass of chromium, and then saying: moles of chromium = m/M' = 2(m_Cr)/(140) ? If so, that method is invalid, which is why you'd have been getting the wrong answer.

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    Supreme Member Flop21's Avatar
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    Re: HSC Chemistry Marathon 2016

    Quote Originally Posted by InteGrand View Post
    The answer is 2*m/M = 2*140/(151.9904) (explanation given in my previous post).
    Oh thanks, I read over your post again more thoroughly and now I understand.
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    Re: HSC Chemistry Marathon 2016

    Quote Originally Posted by InteGrand View Post
    What was the method you were using before? I think you were treating molar mass as M' = 2m_Cr, where m_Cr is the molar mass of chromium, and then saying: moles of chromium = m/M' = 2(m_Cr)/(140) ? If so, that method is invalid, which is why you'd have been getting the wrong answer.
    Yeah I wasn't understanding why you were using the molar mass of the whole chemical, rather than just 2*Cr molar mass. But I understand why you do that now from reading over your post.
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    Re: HSC Chemistry Marathon 2016

    Calculate the mass of copper(I) sulfide [CU2S] which contains 4.6 x 10^23 copper atoms.


    Can someone take me through this one. Finding it super difficult.
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    Re: HSC Chemistry Marathon 2016

    Quote Originally Posted by Flop21 View Post
    Calculate the mass of copper(I) sulfide [CU2S] which contains 4.6 x 10^23 copper atoms.


    Can someone take me through this one. Finding it super difficult.
    I'm lazy again, so I'll do it algebraically for now (I'm essentially doing the general case).

    Let the required mass be m. Let the molar mass of Cu2S be M (you can find this value online or calculate it from a Periodic Table). Let N = 4.6 * 1023. So our goal is to find m in terms of M and N (which we know the values of).

    Now, we each mole of copper(I) sulfide contains 2 moles of copper. Hence there'll be N copper atoms exactly when there are N/2 moles of copper(I) sulfide.

    So we need N/2 = m/M (using "number of moles = mass/(molar mass)", a standard HSC Chemistry formula). Rearranging, we get m = MN/2. Now just plug in the values for M and N to get the answer.

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    Re: HSC Chemistry Marathon 2016

    Can someone help me with this one?

    A student conducting this experiment weighs out 2.40 g of CoCl2.6H2O.

    For this part you will be calculating the amount of sodium carbonate required for the complete precipitation of cobalt(II) carbonate.

    Calculate the moles of CoCl2.6H2O used in this reaction.

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    Ancient Orator leehuan's Avatar
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    Re: HSC Chemistry Marathon 2016

    Quote Originally Posted by BlueGas View Post
    Can someone help me with this one?

    A student conducting this experiment weighs out 2.40 g of CoCl2.6H2O.

    For this part you will be calculating the amount of sodium carbonate required for the complete precipitation of cobalt(II) carbonate.

    Calculate the moles of CoCl2.6H2O used in this reaction.
    They would have to provide you with the equation in the HSC because the hexahydrate will screw up the students' mind.

    Care to show some theory relating to how to generate the equation?

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    Re: HSC Chemistry Marathon 2016

    Quote Originally Posted by leehuan View Post
    They would have to provide you with the equation in the HSC because the hexahydrate will screw up the students' mind.

    Care to show some theory relating to how to generate the equation?
    This isn't a HSC question, it's a question I'm doing for chemistry at uni.

    But here's the equation I'm assuming it meant to be used?

    Na2CO3 + CoCl2.6H2O ===> 2NaCl + CoCO3 + 6H2O

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    Re: HSC Chemistry Marathon 2016

    Yeah I figured you were in uni so that's why I asked for the equation. Which makes sense, I think.

    Then, the mass of CoCl2.6H2O is 2.40g. This is basically the mass of cobalt(II) chloride hexahydrate (the whole thing)
    You can use your university periodic table sheet (or whatever is relevant) to determine the molar mass M.

    The formula is then simply n=m/M?

    (Sorry, if I actually tried it out it seems so trivial. I mean it doesn't look like we have a limiting reagent here.)

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    Re: HSC Chemistry Marathon 2016

    Quote Originally Posted by leehuan View Post
    Yeah I figured you were in uni so that's why I asked for the equation. Which makes sense, I think.

    Then, the mass of CoCl2.6H2O is 2.40g. This is basically the mass of cobalt(II) chloride hexahydrate (the whole thing)
    You can use your university periodic table sheet (or whatever is relevant) to determine the molar mass M.

    The formula is then simply n=m/M?

    (Sorry, if I actually tried it out it seems so trivial. I mean it doesn't look like we have a limiting reagent here.)
    Yeah it seems like I just need to use n=m/M but why would the question include "For this part you will be calculating the amount of sodium carbonate required for the complete precipitation of cobalt(II) carbonate." When it's just as simple as converting grams to moles to answer the question.

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    Re: HSC Chemistry Marathon 2016

    Question: Is pure water acidic, basic or neutral? Justify your claims.

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    Re: HSC Chemistry Marathon 2016

    Quote Originally Posted by BlueGas View Post
    Yeah it seems like I just need to use n=m/M but why would the question include "For this part you will be calculating the amount of sodium carbonate required for the complete precipitation of cobalt(II) carbonate." When it's just as simple as converting grams to moles to answer the question.
    Is it like, a follow-up question?

    Quote Originally Posted by parad0xica View Post
    Question: Is pure water acidic, basic or neutral? Justify your claims.
    Obviously not answered according to HSC student standards:

    Pure water has a pH of 7, but this just means that [H3O+]=10^-7. According to the equation for self ionisation of water 2 H2O(l) <=> H3O(+) + OH(-), the water constant Kw = 1.0*10^-4 (mol L^-1)^2 = [H3O+][OH-]. Direct substitution of [H3O+] shows that [OH-]=10^-7. If [H3O+]=[OH-], then the substance is neutral.

    The self ionisation of water does indeed imply that the concentration of the hydronium ion and the hydroxide ion is not negligible; it is indeed a small number. It is strictly the fact that their concentrations are equal (implying that they cancel out when considering acidity/basicity) that resort in the final answer.

    Note that pure water is actually defined to have a pH of 7. Also note that when we measure the concentrations of aqueous solutions, generally we actually refer to the concentration of such substances in water itself.

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    Re: HSC Chemistry Marathon 2016

    Quote Originally Posted by leehuan View Post
    Is it like, a follow-up question?



    Obviously not answered according to HSC student standards:

    Pure water has a pH of 7, but this just means that [H3O+]=10^-7. According to the equation for self ionisation of water 2 H2O(l) <=> H3O(+) + OH(-), the water constant Kw = 1.0*10^-4 (mol L^-1)^2 = [H3O+][OH-]. Direct substitution of [H3O+] shows that [OH-]=10^-7. If [H3O+]=[OH-], then the substance is neutral.

    The self ionisation of water does indeed imply that the concentration of the hydronium ion and the hydroxide ion is not negligible; it is indeed a small number. It is strictly the fact that their concentrations are equal (implying that they cancel out when considering acidity/basicity) that resort in the final answer.

    Note that pure water is actually defined to have a pH of 7. Also note that when we measure the concentrations of aqueous solutions, generally we actually refer to the concentration of such substances in water itself.
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    Re: HSC Chemistry Marathon 2016

    Quote Originally Posted by Flop21 View Post
    Calculate the mass of copper(I) sulfide [CU2S] which contains 4.6 x 10^23 copper atoms.


    Can someone take me through this one. Finding it super difficult.
    yo flop, is the answer 607.9226171 g = 610g (2 sig fig)?
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    Re: HSC Chemistry Marathon 2016

    Quote Originally Posted by leehuan View Post
    Is it like, a follow-up question?
    Yeah it is, the questions are in this order:

    Identify the net ionic equation for:

    1. CoCl2.6H2O is dissolved in water
    2. The cobalt chloride solution is added to an aqueous solution of sodium carbonate
    3. The cobalt carbonate preciptate is dissolved in excess sulfuric acid
    4. The CoSO4.6H2O crystals are formed from solution after evaporation
    5. Then it's the calculating the moles question I posted earlier

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    Re: HSC Chemistry Marathon 2016

    Quote Originally Posted by Occupied View Post
    yo flop, is the answer 607.9226171 g = 610g (2 sig fig)?
    61g apparently
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    Re: HSC Chemistry Marathon 2016

    Quote Originally Posted by Occupied View Post
    yo flop, is the answer 607.9226171 g = 610g (2 sig fig)?
    You appear to be off by a factor of 10 compared to Flop21's reported answer. What method did you use? Did you try the method I posted (I have t actually typed numbers into a calculator but the method should be right)?
    Occupied likes this.

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