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Percent yield questions

  1. May 8, 2013 #1
    % yield questions (URGENT)

    1. The problem statement, all variables and given/known data
    We have 3 equations:
    2w1tzcy.jpg
    1.64 g of an impure NaNO3-containing substance is reacted with Devarda's alloy. The amount of NH3 got from this reaction is reacted with 25cm3 1.00 moldm-3 H2SO4. The H2SO4 left over is reacted with 16.2 cm3 2.00 moldm-3 NaOH. Calculate the percentage yield of NaNO3 in the impure substance.


    2. The attempt at a solution

    My attempt at an answer:
    1. Amount of H2SO4 reacted with NaOH = (2.00 x 16.2 x 10-3) / 2 = 0.0162 moles.
    2. Amount of H2SO4 reacted with NH3 = 0.025 - 0.0162 = 0.0088 moles.
    3. Amount of NH3 reacted = (0.0088 x 2) = 0.0176 moles.
    4. Amount of NaNO3 reacted = 0.0176 x (3/2) = 0.0264.
    5. Mass of NaNO3 reacted = 0.0264 x 85 = 2.244 g.

    % yield = 2.244/1.64 = 137 %.

    It's wrong because the final mass can't be more than the initial. HELP!!!
     
  2. jcsd
  3. May 8, 2013 #2

    Borek

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    Staff: Mentor

    Why 3/2?

    % yields over 100% do happen. They usually mean something is wrong, but it is not necessarily a math error. For example in this case it could mean that the impurity is some other nitrate.
     
  4. May 8, 2013 #3

    DrClaude

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    Staff: Mentor

    The error is in the line:
     
  5. May 8, 2013 #4

    DrClaude

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    Staff: Mentor

    Edit: But Borek already told you that...
     
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