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    (Original post by Dr00n)
    ahh i don't have the heinneman book
    Ah that might explain it...there's not an awful lot to know on it if that makes you feel any better!
    The oxonium ion is basically the ion that is thought to cause acidity - H+ from acids react with water to form H3O+, so by writing H+ in ionic equations, you're actually referring to H3O+, although H+ is written instead because it is easier
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    Hey folks, hoping you could help me with a question? I can't find a written out method in my textbooks...

    25.0cm3 of 0.100moldm-3 HCl is titrated against 0.09moldm-3 NaOH.

    a) Determine the equivalence point
    b) Calculate the pH at the equivalence point
    c) Work out the pH when an extra 1cm3 of the alkali is added after the equivalence point.

    Cheers
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    Can anyone explain, part (ii):
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    Here is the answer:
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    How to get the answer in part (ii)?
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    Here is the answer:
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    (Original post by Namod)
    Can anyone explain, part (ii):
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    Here is the answer:
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    How to get the answer in part (ii)?
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    Hi there

    With respect to the first question you posted:
    -20 volumes of oxygen are produced from each volume of H2O2 solution.
    -As they are measuring it in 'volumes', you could consider one volume to be 1 mol dm-3.
    -So, 20 mol dm-3 O2 produced for every 1 mol dm-3 of H2O2 reacted.

    -If 20 volumes of O2 is 20 mol dm-3, you can calculate the moles of it from the formula:

    mol = volume/24 (volume must be in dm-3)

    moles of O2 = 20/24
    moles of O2 = 5/6

    From the equation you derived in part i), 2 moles of H2O2 react to produce 1 mole of O2.

    2 x 5/6 = 5/3
    5/3 = 1.67 (3.s.f).

    With respect to the second question you posted:

    -2 moles of glucose are produced from every mole of maltose reacted.
    -Due to molar ratios, rate of formation of glucose will be twice the rate of disappearance of maltose.

    2 x 0.024 = 0.048 mol dm-3 s-1.

    I hope that all made sense, if you have any questions feel free to ask
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    (Original post by nerevs)
    Hey folks, hoping you could help me with a question? I can't find a written out method in my textbooks...

    25.0cm3 of 0.100moldm-3 HCl is titrated against 0.09moldm-3 NaOH.

    a) Determine the equivalence point
    b) Calculate the pH at the equivalence point
    c) Work out the pH when an extra 1cm3 of the alkali is added after the equivalence point.

    Cheers
    Hey

    I will try my best to help you - but do you happen to have the answers to the question you posted?
    I have an answer down but I'm not sure if it's correct, and I wouldn't like to confuse you by giving you the wrong answer, or explaining it incorrectly!

    Thank you!
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    (Original post by zef1995)
    Hey

    I will try my best to help you - but do you happen to have the answers to the question you posted?
    I have an answer down but I'm not sure if it's correct, and I wouldn't like to confuse you by giving you the wrong answer, or explaining it incorrectly!

    Thank you!
    Hi, no sorry I don't have an answer at the moment, they're just ones my teacher made up I think :/ I'll ask her for the answers tomorrow though.

    Go ahead with the answer, it'd be great to see how you'd go about doing it, thanks
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    (Original post by nerevs)
    Hi, no sorry I don't have an answer at the moment, they're just ones my teacher made up I think :/ I'll ask her for the answers tomorrow though.

    Go ahead with the answer, it'd be great to see how you'd go about doing it, thanks
    That's fine, don't worry about it! I've been trying to do b) and c), although I'm not sure how to go about it! If you don't mind me asking, what exam board are you on? I'm just thinking if we're on different boards that may be why I can't do b) or c) :P

    That aside, I think I have an idea of how to do a):

    -In a titration, the equivalence point will be the point where the volume of one solution has reacted exactly with the volume of the second solution (in terms of molar quantities).

    We know there's 25cm3 of HCl, so we can calculate the moles present:
    0.100 x 25/1000 = 2.5 x 10-3

    As HCl and NaOH are in the same molar ratio in the chemical equation, 2.5 x 10-3 moles NaOH will react exactly with 2.5 x 10-3 moles HCl.

    Now to calculate the volume of NaOH used:

    -we know that the concentration of NaOH is 0.090 mol dm-3

    moles/concentration = volume

    (2.5x10-3)/0.090 = 0.02777...dm-3 = 0.278 dm3

    This equates to 27.8 cm3.

    I'm not sure if that's right, I may have made an error somewhere! Although it seems logical in my head, and the answer looks reasonable, I may be wrong :P
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    Hey could someone please look at 4b in this specimen paper: http://www.ocr.org.uk/Images/81033-u...s-specimen.pdf

    I don't understand why Calcium is going from a solid, to a gas, and then back to a solid?
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    (Original post by FailedMyExams)
    Hey could someone please look at 4b in this specimen paper: http://www.ocr.org.uk/Images/81033-u...s-specimen.pdf

    I don't understand why Calcium is going from a solid, to a gas, and then back to a solid?
    They made a mistake, just want to caution you there are lots of mistakes on the specimen paper
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    I got stuck in 2 questions;

    Question 1: Stuck on part (iii)
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    Answer 1:
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    Question 2: on part (ii) how they got the 3.44 mol, I cant get 1.89 mol
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    Answer 2:
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    (Original post by Namod)
    I got stuck in 2 questions;

    Question 1: Stuck on part (iii)
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    Answer 1:
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    Question 2: on part (ii) how they got the 3.44 mol, I cant get 1.89 mol
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    Answer 2:
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    Question 1: If the concentration is 0.01moldm^-3 in 10cm^3 of water, it will be 10x less concentrated in 100cm^3 of water, so just divide the original concentration by a factor of 10 to get the new concentration (0.001moldm^-3). It's a strong acid so the H+ concentration is the same as the acid concentration so just use -log(0.001) and you get the answer.


    Question 2: You have to use the equation : I2 + H2S --> 2HI + S

    Find the moles of I2 (480/126.9 x 2) = 1.89 looking at the equation, 1 mole of I2 should produce 2 moles of HI, so there should be 1.89 x 2 or 3.78 moles produced. This is the theoretical yield.

    Find the actual moles of HI produced (440/127.9) = 3.44, this is the actual yield


    The percentage yield = (actual yield/theoretical yield) x 100

    = (3.44/3.78) x 100 which gives you the answer, 91%
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    Today I did the Jan 2013 paper as a mock, it was a disaster, I left so many out, I really hope it isn't that hard in June, I really need an A :/
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    (Original post by bluedate)
    Today I did the Jan 2013 paper as a mock, it was a disaster, I left so many out, I really hope it isn't that hard in June, I really need an A :/
    I did mine yesterday, Ill be lucky if I got over 50% so so hard :/ hopefully the june paper will be like the previous ones and manageable, if not then goodbye A and offer :/
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    I am resitting this after getting a lovely E in the paper in Jan
    I was quite confident when I went in and then 2 topics (which were across 3 questions) came up that I ran out of time to go over properly so it failed from there...
    I'm pretty sure I can get my B this time, I should still get a B overall if I get a C though (if I do alright in F322 and F324). I've got 35/40 on my coursework which was 1 mark under an A last year, so 46 ums I think.
    I need this B to get into uni!
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    (Original post by bluedate)
    Today I did the Jan 2013 paper as a mock, it was a disaster, I left so many out, I really hope it isn't that hard in June, I really need an A :/
    I did mine yesterday, Ill be lucky if I got over 50% so so hard :/ hopefully the june paper will be like the previous ones and manageable, if not then goodbye A and offer :/
    (Original post by Namod)
    Can anyone explain, part (ii):
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    Here is the answer:
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    How to get the answer in part (ii)?
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    Here is the answer:
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    hi,
    kjust wondering where you got these questions from? thank you!
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    i need an A in this exam! got an A last year or AS overall but got a C in my f324 in january so im resitting! not really that keen on this unit though and for me to get an A this year just seems impossible
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    (Original post by Gulzar)
    How are your AS scores out of 300 ums?
    i got 241 last year.. just scraped an A lol. you?
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    (Original post by narli)
    i got 241 last year.. just scraped an A lol. you?
    I got 247 but a D in f324 so need to retake and aim for that A
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    (Original post by narli)
    i need an A in this exam! got an A last year or AS overall but got a C in my f324 in january so im resitting! not really that keen on this unit though and for me to get an A this year just seems impossible
    Getting a C means that you know your chemistry but you just need to focus on your exam technique and prevent yourself from making those costly silly mistakes
    Just work really hard and I'm sure an A is well within your reach!
    What did u get in the coursework out of 40?
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    (Original post by Gulzar)
    Getting a C means that you know your chemistry but you just need to focus on your exam technique and prevent yourself from making those costly silly mistakes
    Just work really hard and I'm sure an A is well within your reach!
    What did u get in the coursework out of 40?

    A-level requires ALOT of exam technique! So true! We can do it, if we just keep going and don't give up.
 
 
 
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