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Homework Help: Stoichiometric reactions with weight percent

  1. Feb 17, 2012 #1
    i can't figure out how to calculate this. Please help!

    How many grams of 6.000 wt% aqueous HF are required to provide a 50% excess to react with 10.0 mL of 0.0236 M Th4+ by the following reaction?

    Th(4+) + 4F - → ThF4 (s)

    I found the molarity for Th(4+) to be 2.36*10^-4.. I'm not really sure where to go from here =/
     
  2. jcsd
  3. Feb 17, 2012 #2

    Borek

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    Staff: Mentor

    How many moles of F- do you need? What mass of HF will contain this number of moles?
     
  4. Feb 17, 2012 #3
    i tried to solve it again..
    i came up with 0.627g. is this right?
     
  5. Feb 17, 2012 #4

    Borek

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    Staff: Mentor

    Show how you got it.
     
  6. Feb 17, 2012 #5
    using the moles of Th, i multiplied that by 4 to find the number of moles of HF, which turns out o be 9.44*10^-4. Then i changed it to grams, giving me 1.88*10^-2g. since it said 50% excess, the total grams of HF would be 0.376g. then 6 %wt would be 0.627g
     
  7. Feb 17, 2012 #6

    Borek

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    Staff: Mentor

    OK up to now.

    You ate zero, but I think I know what you did. 50% excess and 100% excess are two different things.
     
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