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Homework Help: Dilution Stoichiometry

  1. Nov 19, 2017 #1
    1. The problem statement, all variables and given/known data

    upload_2017-11-19_16-50-24.png
    2. Relevant equations

    M = n/L



    3. The attempt at a solution

    My answer is A) 1.02 M but im not sure if i did this right. Can someone explain why this might be right or wrong?

    I got moles of Al(NO3)3 =0.0393 moles
    I got moles of Ca(NO3)2 = 0.0426 moles by multiplying liters and molarity

    then i added the moles of NO3 in each of them . So for Al(NO3)3 , the mole ration is 1/3 so i get that there is 0.1179 moles NO3 from that compound.

    then i added the NO3 from Ca(NO3)2 using the same ratio so i get 0.0852.

    I added these moles of NO3 together and divided by total liters of solution and got 1.02 M


    My question is am i suppose to treat each compound as it were to break into nitrates and use the mole ratios on each of them to find the amt of nitrates they break down into? I wasn't sure! At first, i thought i had to find a limiting reactant and use only one compound to calculate nitrate. upload_2017-11-19_16-50-24.png
     
  2. jcsd
  3. Nov 19, 2017 #2

    Borek

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    Staff: Mentor

    Your answer is a correct one.

    Limiting reagent is a concept related to the reaction. Is there any reaction taking place?
     
  4. Nov 19, 2017 #3
    Now that you ask. I don't think so because they both stay soluble. None of it precipitated into a solid
     
  5. Nov 19, 2017 #4
    So both of the compounds formed dissociate right? So we get no3 from both the compounds
     
  6. Nov 19, 2017 #5

    Borek

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    Staff: Mentor

    No reaction, no limiting reagent. You just mix two things together. It happens that they both contain the same ion, so this particular ion amount is a sum of amounts that were present in original solutions. And that's how you solved the problem.
     
  7. Nov 19, 2017 #6
    Is there no reaction because the compounds end up being soluble when o wrote the products?
     
  8. Nov 19, 2017 #7

    Borek

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    Staff: Mentor

    There is no reaction because nothing changes - the ions remain exactly as they were before mixing of the solutions.
     
  9. Nov 19, 2017 #8
    oh. Since the keyword in the problem is "mixing", nothing happens right? Or am i missing something?
     
  10. Nov 19, 2017 #9
    Ok... after thinking a bit about this more. The solutions when mixed create 2 aqueous solutions. Which means that both dissociate into the ions separately. So the amount of nitrate is just added from both the compounds.

    However, if it had created a solid, there is a reaction and that means there would be a limiting reactant because only so much of that solid can be form. What's leftover would be the concentration of nitrate of there was a reaction.

    Haha. I'm just writing this down just so I can write down my thoughts!
     
  11. Nov 20, 2017 #10

    Borek

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    Staff: Mentor

    Yes, but (in general) possible reactions are not limited to a solid being created. Things in the solution can combine producing another soluble product, they can react producing gas and so on.
     
  12. Nov 20, 2017 #11
    Wait I'm still confused. Did the reactant go through double displacement then?
     
  13. Nov 20, 2017 #12

    Borek

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    Staff: Mentor

    In this case there was no reaction, period.

    What I am referring to is that you listed a precipitate as if producing an insoluble salt was the only possible way of things reacting. This is actually only one of many possible ways.
     
  14. Nov 20, 2017 #13
    So whenever I predict the products by doing double displacement, I have to check if a reaction occurs? Otherwise they stay in the original form?
     
  15. Nov 20, 2017 #14

    Borek

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    Staff: Mentor

    Aren't you putting a cart before the horse? How can you predict products of the reaction that doesn't occur?

    I guess what you mean here is that you are trying to predict whether a reaction occurs by combining ions and checking whether they can react with each other. That's OK., but checking whether they react is not equivalent of saying they do react.

    Yes, if they don't react they - quite predictably - stay as they are.
     
  16. Nov 20, 2017 #15
    Thank you. I didn't realize that until you mentioned it. Thanks!
     
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