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Homework Help: Simple Concentration Stoic Problem!?

  1. Nov 9, 2017 #1
    1. The problem statement, all variables and given/known data

    What mass of aluminum nitrate should be dissolved in 225 ml of water to make a solute with a total nitrate concentration of 0.866 M ?
    2. Relevant equations

    M = n/L

    3. The attempt at a solution
    So, i keep getting the wrong answer. THe answer is suppose to be 13.8 g but i get a different number
    :

    .226 L * (0.866 mols/L) = 0.19485 mols needed

    Nitrate molar mass is just 16*3 + 14 + 26.98= 88.98 g/mol

    0.19485 * 62 = 17.33 grams of Aluminum nitrate
     
  2. jcsd
  3. Nov 9, 2017 #2

    Charles Link

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    The chemical formula for aluminum nitrate is ## Al (NO_3)_3 ##. I think that might solve your problem.
     
  4. Nov 9, 2017 #3
    I get that the molar mass for (NO3)3 is 186 but i still get 40 g of Al(NO3)3
     
  5. Nov 9, 2017 #4

    Charles Link

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    The molar mass of ## Al (NO_3)_3 ## is 213. Meanwhile, how many moles do you need? With ## (NO3)_3 ## you don't need .195 moles.
     
  6. Nov 9, 2017 #5
    isnt the nitrate concentration refering to just nitrate?
     
  7. Nov 9, 2017 #6

    Charles Link

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    But for every mole of ## Al(NO_3)_3 ## you get 3 moles of ## (NO_3)^- ## in the solution. You do need .195 moles of ## (NO_3)^- ##. That part you got correct.
     
  8. Nov 9, 2017 #7
    Oh my goodness! I keep forgetting you can just get mole-mole ratio from the molecule itself... thanks!
    I spent like so long on this problem ._.
     
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