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OCR chemistry practical speciman help!

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    Got my as chemistry practical next week on OCR. Really stuck on it, dont know how to work out percentage yeild questions on it, or how to work out how the expected volume of CO2 is greater than that is obtained. Looked at the mark scheme, but its not much help as it only gives the final answers but not any of the working required to get the answers! below is the question.

    Procedure
    A 50.0 cm3 sample of 0.100 mol dm–3 sulfuric acid was measured using a pipette. This was
    added to the flask with the side arm.
    0.30 g of magnesium carbonate, MgCO3, was accurately weighed on a digital balance.
    The 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 quickly added and the
    bung replaced.
    Bubbles of carbon dioxide were seen collecting in the measuring cylinder.
    When no more gas was seen to collect in the measuring cylinder, the volume of gas collected
    was recorded as 64 cm3.

    Analysis
    (a) Construct a fully balanced equation for the reaction between H2SO4 and MgCO3, including
    state symbols.

    H2SO4 AQ) + MGCO3 (S) --> MGSO4 (AQ) + H20 (L) + CO2 (G) C1 [1]

    (b) Calculate the number of moles of H2SO4 and MgCO3 used in the experiment.


    moles H2SO4 = 0.005 mol........... C1[1]

    (dont know about this one - i got 0.00356 but mark scheme says its 0.0298!)

    moles MgCO3 =.......................mol ........... C1[1]


    (d) Show by calculation that the expected volume of carbon dioxide is greater than the 64 cm3
    obtained.
    Assume that one mole of CO2 occupies 24.0 dm3 under the conditions used in the
    experiment.

    (also dont understand this one!)

    http://www.ocr.org.uk/qualifications...8/chemistry_a/

    I cant be bothered to post all the rest of the questions, so if anybody can help me with the percentage yeild ones its under the speciman assessment materials at the bottom of that link ^^
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    Percentage Yield = (Actual Yield/Theoretical Yield) x 100

    That of any use?
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    i really dont get how you get 71.4 for the following question:
    Show by calculation that the expected volume of carbon dioxide is greater than the 64cm3 obtained. I multiplied 3.56*10-3 the moles of MgCO3 by 24000 and got 85cm3
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    also I dont understand the mark scheme for the last questions:
    d) The students decided that the experiment could be carried out more accurately if both the procedure and measuring apparatus were modified.
    i) Draw a labelled diagram to show how the carbonate and acid could be reacted together without needing to remove the bung from the flask.

    e) The same method was used to measure the volume of co2 produced when magnesium carbonate reacted with HCL. Suggest with reasons any modifications you need to make if the sulfuric acid used in the procedure was replaced with the same concentration and volume of hydrochloric acid.
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    (Original post by Nabster365)
    i really dont get how you get 71.4 for the following question:
    Show by calculation that the expected volume of carbon dioxide is greater than the 64cm3 obtained. I multiplied 3.56*10-3 the moles of MgCO3 by 24000 and got 85cm3
    I think I remember this vaguely ..it was MS with a wrong answer ..your answer ringing bells
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    Hmm yes, we also think the Ms is wrong

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Updated: May 11, 2013
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