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    (Original post by Nez)
    if you read the cover sheet of the panl, it only says that u need to show how you determined the quantities you used for the tiratation. you dont actually need to show calc. for working out the number of moles of water, u only need to do that dor the heating method...
    You can't show why a titration helps you determine water of crystallisation without referring to the maths and calculations you would use. You can't just go.. this is my titration.. this is what I'm going to use.. and it tells me no.moles water. How? You can only show how the results would determine no.moles water by outlining the calculations and the principles you'd base them on.
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    (Original post by mubarak)
    :eek: Urgent help neeed please!!
    need to refer to the thermal stability of sodium carbonate.
    No idea what to say.
    Any ideas?

    Cheers
    Mubarak, try this: 2. http://www.chemguide.co.uk/inorganic...compounds.html
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    (Original post by mubarak)
    Do u think id have to do enthalpy cycles as part of the calculation?
    cheers
    Good heavens NO!

    Read back for the discussion on thermal decomp. The stuff spits when heated.. so you wouln't want burning crystals scattered over you and in your eyes (even if the lid is on.. you have to take it off at some point!)
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    Wrong craghyrax. Pg 31, 33-34, 180. OCR Chemistry 1.

    Just mention enthalpy change (exothermic or enothermic) and back up your answer with proof from the pages on lattice structure.

    not do any A2 level enthalpy hess law diagrams. unless you want to
    Hess law is only mentioned briefly at AS (pg 155-159)
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    (Original post by tozhan)
    Wrong craghyrax. Pg 31, 33-34, 180. OCR Chemistry 1.

    Just mention enthalpy change (exothermic or enothermic) and back up your answer with proof from the pages on lattice structure.

    not do any A2 level enthalpy hess law diagrams. unless you want to
    Hess law is only mentioned briefly at AS (pg 155-159)
    Lattice enthalpy? Maybe I'm a bit foggy but don't remember coming across lattice enthalpy at AS.. remember formation..combustion..general enthalpy stuff. Well fair enough.
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    Thanks guys its really useful especially the page numbers shud have this finished tonight and revise a bit 2moro

    Good luck everyone!!
    :party:
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    soz can i jst ask this..
    how much of hydrated sodium carbonate has everyone mentioned?
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    NEED TO KNOW!!!
    In the actual practical exam are we allowed to take our plan in and look at in in the exam?!
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    i dont think so, may i might be wrong
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    Right yes, maybe I've left this a little late *tomorrow*
    But I'm really stuck on the calculations for thermal decomposition.

    Well, specifically one part of them.

    My friend's showing me her method, and she's started off like this:

    Mass of anhydrous sodium carbonate, Na2CO3:

    Crucible + lid + hydrated sodium carbonate = 26.25g
    Crucible + lid + Na2CO3 after second heating = 25.0g
    26.25 - 25.0g = 1.25g

    but I'm really confused because lol doesnt that just give you the mass lost? She gets the right results and stuff though, and as I may have to be doing this tomorrow I want to know how you do it!

    I would have thought it was something like... mass of crucible + lid + residue - mass of crucible + lid.

    but that doesnt work

    and i have no idea where she got her figures from either
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    (Original post by mubarak)
    NEED TO KNOW!!!
    In the actual practical exam are we allowed to take our plan in and look at in in the exam?!
    Yes we ARE allowed to take our planning into the prac exam. AND look at it. However.. I don't think I glanced at it more than once in my prac exam last year.. Not enough time! Its reassuring though...
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    (Original post by at_amber)
    Right yes, maybe I've left this a little late *tomorrow*
    But I'm really stuck on the calculations for thermal decomposition.

    Well, specifically one part of them.

    My friend's showing me her method, and she's started off like this:

    Mass of anhydrous sodium carbonate, Na2CO3:

    Crucible + lid + hydrated sodium carbonate = 26.25g
    Crucible + lid + Na2CO3 after second heating = 25.0g
    26.25 - 25.0g = 1.25g

    but I'm really confused because lol doesnt that just give you the mass lost? She gets the right results and stuff though, and as I may have to be doing this tomorrow I want to know how you do it!

    I would have thought it was something like... mass of crucible + lid + residue - mass of crucible + lid.

    but that doesnt work

    and i have no idea where she got her figures from either
    Ok. She had it right. The mass leftover is the mass lost on heating. The mass lost is the mass of water of crystallisation because na2co3 has a melting point so high that it doesn't break down at bunsen burner temp. Now you know the mass in grams of water lost.. you divide it by your relative molecular mass (of water) to tell you how many moles of water were in the sample. You know the mass of Na2CO3 remaining.. you can divide it by its relative molecular mass to tell you how many moles of that you have. Work out the equation from there.
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    (Original post by Craghyrax)
    Ok. She had it right. The mass leftover is the mass lost on heating. The mass lost is the mass of water of crystallisation because na2co3 has a melting point so high that it doesn't break down at bunsen burner temp. Now you know the mass in grams of water lost.. you divide it by your relative molecular mass (of water) to tell you how many moles of water were in the sample. You know the mass of Na2CO3 remaining.. you can divide it by its relative molecular mass to tell you how many moles of that you have. Work out the equation from there.
    Thats where I have gotten stuck, I realise if you do e.g
    0.4/18 = 0.02
    so thats the not moles present at all......thats where I was forgetting...

    If you work out a ratio.... (what I've tried to do) I get stuck, anyone help me out here with number crunching!?
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    (Original post by -sophie-)
    Thats where I have gotten stuck, I realise if you do e.g
    0.4/18 = 0.02
    so thats the not moles present at all......thats where I was forgetting...

    If you work out a ratio.... (what I've tried to do) I get stuck, anyone help me out here with number crunching!?
    No. Molecular/atomic mass = mass of 1 mole in grams. So. 18g = mass of 1 mole of water. That may be how many moles of water you have in your sample. Stop looking at the water and figure out the carbonate. After you've got rid of the water figure out how many moles of the carbonate you have. Then you divide the mass of water lost by the mass of carbonate to figure out how much water was attached to each mole of carbonate. Then divide that mass by mr to get no moles. And trust that its right. Anyway.. how can you get it wrong? You don't have to DO it.. just explain how you'll do it. Let 'n' be the value of carbonate and describe what you'll do to calculate it. You don't have figures until you've done it in real life.. which they're not asking.
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    (Original post by Craghyrax)
    Ok. She had it right. The mass leftover is the mass lost on heating. The mass lost is the mass of water of crystallisation because na2co3 has a melting point so high that it doesn't break down at bunsen burner temp. Now you know the mass in grams of water lost.. you divide it by your relative molecular mass (of water) to tell you how many moles of water were in the sample. You know the mass of Na2CO3 remaining.. you can divide it by its relative molecular mass to tell you how many moles of that you have. Work out the equation from there.
    i dont think you do. thats the problem.

    if you have no starting mass before you take the water away you will not be able to work it out because the mass of water removed depends on the ratio (H2O:Na2CO3)

    1.25g of water removed goves 0.06944 moles H2O.
    you need to know moles Na2CO3 (as you said) but we dont have a mass to work with. before or after heating!
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    (Original post by tozhan)
    i dont think you do. thats the problem.

    if you have no starting mass before you take the water away you will not be able to work it out because the mass of water removed depends on the ratio (H2O:Na2CO3)

    1.25g of water removed goves 0.06944 moles H2O.
    you need to know moles Na2CO3 (as you said) but we dont have a mass to work with. before or after heating!

    Ok. Say you have 150g of HYDRATED sodium carbonate (unknown ratio). You drive off the water. Then you know that what you have left is only sodium carbonate. You weigh that. Divide mass by Mr of sodium carbonate. Then you have no moles sodium carbonate. Ok. Now you divide mass water driven off by Mr of water. This gives you no moles water. Now you divide No moles water by no moles sodium carbonate. This gives you no moles water per sodium carbonate mole. Therefore this gives you ratio. So... You do know the mass. Because you weigh your sample before and after the water is driven off. However you do this, you are going to weigh your unknown sample before and after.
    Well.. now that I've just given out the answer...
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    that is all correct but the post asks whether knowing the amount of na2co3 is required to find no. moles na2co3.

    it is.

    the mass must be weighed first by subtracting the mass of crucible from mass of crucible+contents
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    Okay so the test is done now

    What did everyone think about it?
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    totally gay. i messed up the evaluation coz i couldnt think of stuff to write. and i subtracted instead of added during the hess law stuff.
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    werent u suppose to subtract it anyway?

    a - (-b)= +answer
 
 
 
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