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Chem synoptic maths-y questions :'( watch

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    Ok - sorry about this everyone, but I was trying the Edexcel Jan 2003 Synoptic paper and i can't seem to do most of the maths question on it - and i can't see where i am going wrong half the time.

    1a) There are 3 steps to a reaction scheme (propanol --> propanoic acid --> 2-chloropropanooic acid --> Compound D). Asumming the percentage yield for each step in the sequence to be 80%, calculate the mass of D that could be made from 60g of propanol.

    Sally's answer: 30.72 Answer in markscheme: 46.1

    b)An aqueous solution of D of concentration 0.100 mol.dm^-3 has a pH value of 2.04. Calculate the value of the dissoclation constant Ka, for D.

    Sally's answer: 8.32 x 10^-4 Answer in markscheme: 8.32

    2. 2MnO4^- + 16H^+ + 5C2O4^2- -----> 2Mn^2+ +8H2O + 10CO2
    In a particular reaction expt, 200cm^3 of aqueous KMnO4 of conc 0.05 were mixed with 50cm^3 od C2O4^2- of conc 0.5 and 80 cm^3 of conc 1.0 of H2SO4. Show that the starting conc of MnO4^- is 3.03 x 10^-2

    3.Solid MgSO4 conatins water of crystallisation, MgSO4.xH2O. 1.23g of MgSO4 crystals was dissolved in distilled water and excess barium itrate solution was added. The white ppt of BaSO4 was filtered off, dried and weighed. 1.16g of anhydrous BaSO4 was formed. Calculate x.

    The markscheme got 7. I got 7.0589... and rounded off it would give the rigth ans, but did everyone else get a rounded figure? I did it a different method to the markscheme and i was wondering whether my method was wrong.

    Sorry this is so long!!! And if you have read up until the end, i salute you

    Thanks
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    a) 0.8*0.8*0.8*60 = 30.72 :confused: - they're wrong.

    b) i also get 8.32e-4
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    (Original post by elpaw)
    a) 0.8*0.8*0.8*60 = 30.72 :confused: - they're wrong.
    Well i did it that way as well. if it helps, compound D = CH3.CH(OH).COOH

    They did it:

    0.8 x 0.8 x0.8 = 0.51
    1mol of propanol = 60 g 1mol D = 90g
    therefore 0.5 x 90 = 46.1 g :confused:
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    (Original post by Silly Sally)
    Well i did it that way as well. if it helps, compound D = CH3.CH(OH).COOH

    They did it:

    0.8 x 0.8 x0.8 = 0.51
    1mol of propanol = 60 g 1mol D = 90g
    therefore 0.5 x 90 = 46.1 g :confused:
    oh yeah, 30.72g of propanol will react, and so 90/60 * 30.72g = 46.1g of D
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    (Original post by elpaw)
    oh yeah, 30.72g of propanol will react, and so 90/60 * 30.72g = 46.1g of D
    Why have you done 90/60?
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    (Original post by Silly Sally)
    Why have you done 90/60?
    to get the number of moles of propanol, you divide 30.72 by 60. because its a 1-to-1 reaction, you will get that number of moles of D, so the mass of D is that number multiplied by 90. or in short, 90/60.
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    I can help with question 2, but could someone post their workings for question 1 part b cos I can't even get 8.32 to 10^-4 (i get 9.something which is certainly wrong).

    2.

    Starting moles Mn04^- = MV/1000 or cV = (0.05*200)/1000 = 0.01

    Starting concentration = starting moles/total volume = (0.01*1000)/(200+50+80) = 3.03 * 10^-2

    For 3, I haven't done the question, but I wouldn't worry if the answer needs to be rounded, they often do for those questions because of the small number of decimal places we use.


    Please can someone help with 1 b) ?
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    isnt it 90/60 x 60*0.8x0.8x0.8
 
 
 

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