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    (Original post by 76584)
    something confusing me about sterioisomers because my textbook doesnt make it very clear...
    With E/Z isomers i get that you compare atomic numbers
    My question is do you compare the entire atomic number of the whole group to the other side...or just invidivual atoms...for example

    chlorine on one side and CH15(just an example) on the other side
    Would you do Chlorine vs Carbon...chlorine wins....then chlorine vs hydrogen...so chlorine vs 15...chlorine wins so chlorine is the highest priority
    or would you do chlorine vs the entire atomic number of ch15 which would be like 21...which would be higher than chlorine so CH15 is higher priority
    thanks for any help
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    (Original post by Dinaa)
    Need a little help understanding what happens to the ionic radius going across a period.
    I understand that it increases as you go down a group, because of the increase of protons, therefore extra shells (distance factor) and there is more shielding. But what happens across a period?

    I know what happens from e.g. Na- Al. The positive charge increases, therefore positive>negative. Hence, greater attraction between the electrons and the nucleus, making the ionic radius smaller.

    (Please tell me if i'm wrong, or if I've missed any details)

    But then from Si- Cl, the negative: positive charge is greater.. Therefore less attractive?? Idk :cry2:

    Pls help

    EDIT:

    Oh.. It's just because the number of protons increase across a period? Therefore greater nuclear attraction?

    Are ions down a period isoelctronic? Therefore from Na-Cl they are have the same number of electrons?

    OK, so if that is the case, then the protons(+) increase, whilst the electrons(-) stay the same. Causing positive>negative, therefore greater nuclear attraction!

    (please tell me that is right lmao, and if i've missed any details or whatever


    I was gonna add something but the people who replied earlier gave you some good responses
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    Complete combustion of 50 cm3 of a hydrocarbon vapour gave 350 cm3 of carbon dioxide?


    both gas volumes being measured at the same temperature and pressure. The
    formula of the hydrocarbon could be:

    A: C8H18

    B: C7H16

    C: C6H14

    D: C5H12

    Could some one explain the answer?

    I went with my gut instinct and divided 350 by 50. This gave me 7 so I went with B, the answer is correct but I do not understand why???? Has this got something to do with Avogadro??

    please help
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    (Original post by TheonlyMrsHolmes)
    Complete combustion of 50 cm3 of a hydrocarbon vapour gave 350 cm3 of carbon dioxide?


    both gas volumes being measured at the same temperature and pressure. The
    formula of the hydrocarbon could be:

    A: C8H18

    B: C7H16

    C: C6H14

    D: C5H12

    Could some one explain the answer?

    I went with my gut instinct and divided 350 by 50. This gave me 7 so I went with B, the answer is correct but I do not understand why???? Has this got something to do with Avogadro??

    please help
    see your post thread in the forum ..
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    (Original post by Dylann)
    So if you're strong in unit 1 there's no way you're gonna fail! see we're already making progress. I would recommend using this guy:

    www.youtube.com/user/NewcastleChemistry to help! He should be able to help you with energetics unit 2. Make notes and once you think you've understood head over to:

    www.a-levelchemistry.co.uk and select AQA AS and do some of the energetics exercises! This should help reinforce what you've learned.

    For the substance questions, I understand some of them can be pretty trick but use the a-levelchemistry website and try out some questions there tell me how it goes
    Thanks a lot, I'll be sure to check them out!
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    Thought I'd share this online app from RSC's Chemnet Its mainly to help A-Level students understand how to use calculations and how to solve and find answers in a step-by-step method. It really helps since some people lose marks on simple calculations Here's the link
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    Anyone done the AS evaluative assessment for OCR? I'm unsure as to what I should be focussing my time on in terms of revision
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    I have a question about a simple calculation, here's the info I'm given;

    A 50.0 cm3 sample of 0.100 moldm-3 sulphuric acid was measured.

    0.30 g of magnesium carbonate, MgCO3 was weighed.

    A measuring cylinder was filled with exactly 100 cm3 of water and supported upside down in a trough of water.

    The two pieces of apparatus were connected by a delivery tube so that any gas produced in the side arm flask would be collected in the measuring cylinder. The apparatus was airtight and no leaks were present.
    The rubber bung was removed from the side arm flask. The MgCO3 was added and the bung replaced.


    After being asked to write the balanced equation, (H2SO4 (aq) + MgCO3 (s) = MgSO4 (aq) + CO2 (g) + H2O (l) )
    I was asked to calculate the number of moles of H2SO4 and MgCO3, I calculated the number of moles of H2SO4 (0.005 mol) and then assumed that this was the same for the number of moles of MgCO3 as they are in a 1:1 ratio as shown by the equation, however I appear to be wrong, the answer was 0.0035 mol.... why don't I use the ratio like normal? is it because I haven't been given the same info about MgCO3? I don't get it... thank you guys




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    (Original post by Georgiam247)
    I have a question about a simple calculation, here's the info I'm given;

    A 50.0 cm3 sample of 0.100 moldm-3 sulphuric acid was measured.

    0.30 g of magnesium carbonate, MgCO3 was weighed.

    A measuring cylinder was filled with exactly 100 cm3 of water and supported upside down in a trough of water.

    The two pieces of apparatus were connected by a delivery tube so that any gas produced in the side arm flask would be collected in the measuring cylinder. The apparatus was airtight and no leaks were present.
    The rubber bung was removed from the side arm flask. The MgCO3 was added and the bung replaced.


    After being asked to write the balanced equation, (H2SO4 (aq) + MgCO3 (s) = MgSO4 (aq) + CO2 (g) + H2O (l) )
    I was asked to calculate the number of moles of H2SO4 and MgCO3, I calculated the number of moles of H2SO4 (0.005 mol) and then assumed that this was the same for the number of moles of MgCO3 as they are in a 1:1 ratio as shown by the equation, however I appear to be wrong, the answer was 0.0035 mol.... why don't I use the ratio like normal? is it because I haven't been given the same info about MgCO3? I don't get it... thank you guys



    Mol=mass/Mr

    Mass of MgCO3 was 0.3grams and you can work out the Mr

    Spoiler:
    Show
    0.3/84.3 = 0.0035

    Mg weighs 24.3
    O x 3 = 16 x 3 = 48
    C = 12
    24.3+48+12 = 84.3 = Mr


    (You're right that equal moles reacted but you didn't have equal moles to start with)

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    (Original post by Dylann)
    Mol=mass/Mr

    Mass of MgCO3 was 0.3grams and you can work out the Mr

    Spoiler:
    Show
    0.3/84.3 = 0.0035

    Mg weighs 24.3
    O x 3 = 16 x 3 = 48
    C = 12
    24.3+48+12 = 84.3 = Mr


    (You're right that equal moles reacted but you didn't have equal moles to start with)

    Posted from TSR Mobile
    Ahh okay I see, I don't know why I find it such a difficult concept to grasp, but that helps, thank you
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    (Original post by Georgiam247)
    Ahh okay I see, I don't know why I find it such a difficult concept to grasp, but that helps, thank you
    I feel bad that you posted the WHOLE question lol!
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    (Original post by Dylann)
    I feel bad that you posted the WHOLE question lol!
    Hahah well people usually tell me I haven't given enough info so I was playing it safe lol :P
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    (Original post by Georgiam247)
    Hahah well people usually tell me I haven't given enough info so I was playing it safe lol :P
    Actually that's true, a lot of students here don't include vital information which makes it very difficult to help
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    Did anyone do the Edexcel 2014 paper?

    How was it? :afraid:
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    (Original post by zhang-liao)
    Thought I'd share this online app from RSC's Chemnet Its mainly to help A-Level students understand how to use calculations and how to solve and find answers in a step-by-step method. It really helps since some people lose marks on simple calculations Here's the link
    PRSOM, that's great! Thank you :hugs:
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    (Original post by Dylann)
    Actually that's true, a lot of students here don't include vital information which makes it very difficult to help
    Well you helped me just fine! so could you tell me the key to getting an A in Chemistry? Anything you couldn't have done it without?
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    (Original post by Georgiam247)
    Well you helped me just fine! so could you tell me the key to getting an A in Chemistry? Anything you couldn't have done it without?
    Past papers, I do AQA and the questions are often very similar!!
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    (Original post by Dylann)
    Past papers, I do AQA and the questions are often very similar!!
    Oohh I see. Thank you :P
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    Which of the following ions would be deflected most in a mass spectrometer?

    A 35Cl+
    B 37Cl+
    C 37Cl2+
    D (35Cl —37Cl)

    Why is it C? I thought the heaviest is deflected the least.
    I thought A because it'd be 35-1=34, which is the lightest there, therefore deflected the most?
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    (Original post by Dinaa)
    Which of the following ions would be deflected most in a mass spectrometer?

    A 35Cl+
    B 37Cl+
    C 37Cl2+
    D (35Cl —37Cl)

    Why is it C? I thought the heaviest is deflected the least.
    I thought A because it'd be 35-1=34, which is the lightest there, therefore deflected the most?
    Well do you want the chemists answer or the physicists

    As a chemist is the mass per unit charge that counts, so the heavier stuff gets deflected less true but the increased charge matters more here as we want a ratio!

    As a physicist the force acting is proportional to charge and inertia proportional to mass. So according to the definition of an electric field combined with N2L:

    a=Eq/m

    A is acceleration
    E electric field.
    Q charge
    M is mass
 
 
 
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