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Need help, i think

  1. Apr 3, 2009 #1
    1. The problem statement, all variables and given/known data
    hey guys, i got a problem. "concentated nitric acid is 70.7% HNO3 (molar mass=63g) and 29.3% H20 by mass. the density of the solution is 1.42gcm. what is the molarity.


    2. Relevant equations
    i know i should rearrange the formula d=m/v but i cant work out the mass becuase how do i know how many moles of HNO3 do i have? i tried writing empricial formula but im a bit lost. any help would be greatlyfull apprecitated.


    3. The attempt at a solution
    P.s ive got an assesment and i have a few worked questions, if anyone would like to check one or 2 answers please let me know. thanks
     
  2. jcsd
  3. Apr 3, 2009 #2

    Redbelly98

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    Start by working with those mass-% numbers. You may assume any amount of HNO3 and H2O, as long as they are in the right proportion. So to make life simple, use 70.7g of HNO3 and 29.3g of H2O.
     
  4. Apr 3, 2009 #3
    yeh i did that but when i worked it out i get 1.122 moles of HNO3 and 1.623 moles of H20. should i just mulitiple both by 2?
     
  5. Apr 3, 2009 #4

    Redbelly98

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    Not sure what good multiplying by 2 does.

    At any rate, you now have how many moles of HN03. Given the density 1.42gcm (is that 1.42 g/cm3?), you can figure out the volume of the solution.
     
  6. Apr 3, 2009 #5
    so if i times 70.7 grams by 1.122 that should give me the mass of HNO3
     
  7. Apr 3, 2009 #6
    oh soz, yeh it is cm^3
     
  8. Apr 3, 2009 #7

    Redbelly98

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    Uh, no, the mass of HNO3 is 70.7 grams. 70.7g x 1.122mol would be meaningless here.

    The total mass of the solution is 100.0 grams. Use the density to get the volume of the solution.
     
  9. Apr 3, 2009 #8
    o true. so it would work out v=m/d =100/1.42=70.422. then molarity would equal moles/volume which would equal 1.122/70.422=0.0159
     
  10. Apr 3, 2009 #9
    thansk mate your a ledgend.
     
  11. Apr 3, 2009 #10

    Redbelly98

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    You are very very close, hang in there.

    Watch those volume units:
    What are the units for the v=70.4 you calculated?
    What volume units are used to calculate molarity?
     
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