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Determing the mass percent of chloride from a mixture of NaCl and KCl salts

  1. Oct 16, 2012 #1
    Hi guys,

    I am doing a lab experiment which involves analysis of a sample of a mixture of NaCl and KCl. I'm using volumetric titration ( Mohrs and Fajans methods) as well a Gravimetric analysis technique. The underlying idea is determine the mass percent of chloride in the same sample using different techniques to see which one is better. The mass percent of the sample, regardless of the method employed, should be similar but the results I found are extremely off. I'm not sure what Im doing wrong.

    For example:

    For Fajan's method:

    Trial 1: 14.31 mL of silver nitrate were needed to reach equivalence point. So I determine the mass(%) chloride in the following way:

    14.31 mL/ 1000 x 0.1 M Silver Nitrate = 0.00143 moles of Silver Nitrate

    1:1 ratio so the moles of chloride must be equal to the moles of Ag.

    moles(chloride)=0.00143 moles x 35.453 g/mol = 0.050 grams?

    how is that possible, though? the initial mass of the mixture was 0.1083 grams which was used to make a 250 mL solution. I then did three trials of titration using 25 mL aliquots..each 25mL aliquot should contain 0.01083 grams of mixture...how is it that the mass of just chloride alone is more than mass of the entire mixture. (0.050/0.01083) x 100 = 461% mass. this is obviously wrong. i dont get it :S

    In addition, from the gravimetric analysis procedure, I got a mass percent chloride of 58.3. That seems realistic, and I think Fajan and Mohr mass percent should be in that range..not 461% lol. Can anybody help? Thanks!
     
  2. jcsd
  3. Oct 16, 2012 #2

    Borek

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    Staff: Mentor

    Really that low? If I am not mistaken that would mean you were aiming at using less than 2 mL of the titrant, doesn't make any sense to me.
     
  4. Oct 16, 2012 #3
    Sorry, you're right. Its not 0.1083 but 1.0083 grams
     
  5. Oct 17, 2012 #4

    Borek

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    Staff: Mentor

    That puts your results in the same ballpark, doesn't it?
     
  6. Oct 17, 2012 #5
    Yes it does, thank you for pointing this out! wow..one whole day wasted on trying to figure out why i was so dumb..
     
  7. Oct 17, 2012 #6

    Borek

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    Staff: Mentor

    Welcome to the club :wink:
     
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