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Thread: pH question

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    pH question

    Screen shot 2016-03-29 at 1.44.54 PM.png

    Can anyone show working out for (a), (b) and (c)

    I know the pH change for (d) is 0.01 to 1.01

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    Re: pH question

    Thanks in advance!!

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    Re: pH question

    It's a multiple choice answer, it asking which of these would cause the greatest change the CaCo3 is basic NaOH is basic and HCl is acidic, adding the HCl will do nothing, it might increase a little but not much, its between a,b with NaOH being the most basic or Alkaline meaning that b should be the answer. ( I think CaCO3 is marble...)

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    Ancient Orator leehuan's Avatar
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    Re: pH question

    c) is the easiest case. You're mixing two samples with the same pH. That won't change the pH at all.
    pH = -log_10(0.1) = 1

    d) is essentially a dilution. A dilution tenfold trivially increases the pH by one.
    pH = 2

    Proof:
    C1V1=C2V2
    0.1molL^-1 * 100mL = C2 * 1000mL
    0.01molL^-1 = C2
    Conc. HCl = Conc. H3O+
    pH = -log(0.01) = 2

    b) is a neutralisation. HCl + NaOH -> NaCl + H2O
    1:1 molar ratio
    n(HCl) = 0.1 * 0.1 = 0.01mol
    n(NaOH) = 0.01 * 0.1 = 0.001mol
    Hence NaOH is the limiting reagent
    n(HCl) after reaction = 0.01-0.001 = 0.009mol
    Volume after reaction = 110mL
    Hence [H3O+] = [HCl] = 0.009/(110*10^-3) = 0.081818181818181..
    pH = -log_10[H3O+] = 1.08

    a) is the more dodgy one. Also a neutralisation. 2 HCl + CaCO3 -> CO2 + H2O + CaCl2
    The process is similar to b) - use the concept of a limiting reagent. But I'll leave this one alone.

    Of course, to calculate the moles of CaCO3 you need n=m/MM

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    Ancient Orator leehuan's Avatar
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    Re: pH question

    Quote Originally Posted by Jippyjitsu1 View Post
    It's a multiple choice answer, it asking which of these would cause the greatest change the CaCo3 is basic NaOH is basic and HCl is acidic, adding the HCl will do nothing, it might increase a little but not much, its between a,b with NaOH being the most basic or Alkaline meaning that b should be the answer. ( I think CaCO3 is marble...)
    CaCO3 is limestone so yes virtually marble.

    But whilst I do doubt the authenticity of my calculations, if they are correct then you would be surprised to find that because we add such a limited amount of NaOH we cause little harm to the system's pH..

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    Re: pH question

    I have my half yearlies on Thursday. Do you have any tips or have anything to tell me to revise over specifically?

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    Re: pH question

    Quote Originally Posted by RachelGreen View Post
    I have my half yearlies on Thursday. Do you have any tips or have anything to tell me to revise over specifically?
    Revise everything and anything in the first two topics in the syllabus
    Bachelor of Science (Advanced Mathematics) @ USYD

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    Ancient Orator leehuan's Avatar
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    Re: pH question

    ^Well tbh revise anything that has been taught. Some schools will drop out sections in topics if not taught.

    But try looking for some half yearly papers. I'm not sure if dan has any

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    Re: pH question

    Hey can you also help me with this difficult POM question, I know this is the wrong thread to post this in but who cares.

    A scientist placed a rock on top of a few sheets of newspaper, under which was an envelope containing photographic film. After only 5 minutes he removed the photographic film and found that an image in the shape of the rocket had developed on the film. After 28 days, he demonstrated this phenomenon to his colleagues with the same rock, but this time it took 20 minutes for the image to develop to the same intensity.

    This table shows properties of some radioactive isotopes:
    Name of isotope/Radiation emitted/Approximate half-life

    Sodium-24/beta,gamma/15 hours
    Bismuth-210/beta/5 days
    Iodine-131/beta/7 days
    Phosphorous-32/beta/14 days
    Radium-225/alpha/14 days
    Thorium-227/alpha/28 days
    Protactinium-234/beta/28days

    Use information about the scientist's experiment and that in the table above to deduce which of these radioactive isotopes the rock may have contained. Explain your choice. [2]

    I'm able to explain why it is phosphorous-32 in terms of the radiation. The radiation has to be gamma or beta, to penetrate through the objects, but I can't get the second mark to explain in terms of short half life

    Screen shot 2016-03-29 at 8.56.14 PM.png
    And this Jame's Ruse MC question got me stuck, Idk why I'm having so much problems with pH questions

    Thanks to whoever helps me with the exam questions!
    Last edited by RachelGreen; 29 Mar 2016 at 9:07 PM.

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    Ancient Orator leehuan's Avatar
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    Re: pH question

    I can't articulate it carefully but note that initially the decay occurred in 5min

    But then 28 days later the decay occurred in 20min

    Incidentally, the half life of the phosphorus is 14 days
    You can just think about it logically
    Initially - decays over 5min
    14 days later - decays over 10 min
    28 days later - decays over 20 min
    42 days later - decays over 40 min

    etc.

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    Ancient Orator leehuan's Avatar
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    Re: pH question

    I'll just do the method for the pH question

    1. write out equation
    2. determine molar ratio
    3. determine initial moles of Ba(OH)2 and HCl
    4. identify limiting reagent
    5. determine quantity (moles) of excess reagent left
    6. find concentration of excess reagent (note: new volume is 60.0mL)
    7. apply dilutions formula C1V1=C2V2
    8. calculate pH

    Possible step 7.5 - If Ba(OH)2 is the excess reagent, you may need Kw = 1.0*10^-14 = [H3O+]*[OH-]
    Or use pH + pOH = 14

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    Re: pH question

    Oh cool, thanks I got it!

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    Re: pH question

    Wait, hold on what's step 7.5 "Kw = 1.0*10^-14 = [H3O+]*[OH-]"

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    Ancient Orator leehuan's Avatar
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    Re: pH question

    Self ionisation of water

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