Solution Stoichiometry/ Solution Dilution

betterfuture

Full Member
2+ Year Member
Feb 16, 2016
594
96
    What volume of 0.150 M KCl solution will completely react with 0.150 L of a 0.175 M Pb(NO3)2 solution according to the following balanced chemical equation?

    2KCl + Pb(NO3)2 ---> PbCl2 + 2KNO3

    Okay so I was doing solution dilution problems with M1V1=M2V2 which is important for preparing diluted solutions in the lab or during the equivalence point, but then I encountered the above problem. I saw in the book they used a lengthy process to figure out the volume for KCl solution but I used a shortcut method. I used M1V1=M2V2. I of course got a different answer than the book. But I noticed that if I multiplied the answer by 2 I would get 0.350 L KCl(the answer)

    I think I just figured out a shortcut method! I thought I would let others know because the trick is that in M1V1=M2V2 the moles don't change. But here, 1 mole of Pb(NO3)2 reacts with 2 moles of KCl so you would have to use stoichiometric coefficients to get the right answer corresponding to the reaction of the two reactants. I am not sure if this works for every problem, I just worked something out in my brain. Is my reasoning correct? Am I allowed to use this method?
     
    Last edited:

    aldol16

    Full Member
    5+ Year Member
    Nov 1, 2015
    5,300
    4,065
    1. Medical Student
      If I'm interpreting your approach correctly, then yes you are doing it correctly. The point here is that M1V1 = M2V2 is for dilutions where you're not interconverting between substances. In a chemical reaction, you are interconverting between substances and the mole ratios between those substances are not always one. Therefore, you need to correct for that. You did it in your head, but another way to do it is to use normality, or normals of solution. Basically, that takes into account the fact that 1 mole of [Pb] is equivalent to 2 moles of KCl in this reaction.
       

      betterfuture

      Full Member
      2+ Year Member
      Feb 16, 2016
      594
      96
        Oh man! So you mean I can actually use this faster method?

        And I am just wondering, why is it that I never encountered normality is any of my undergrad chemistry courses? Is this a fairly new topic or is it just not important enough to be covered in classes? I ask because Normality kind of bugs me and most likely the MCAT will use Normality in titrations and solution chemistry.
         

        aldol16

        Full Member
        5+ Year Member
        Nov 1, 2015
        5,300
        4,065
        1. Medical Student
          And I am just wondering, why is it that I never encountered normality is any of my undergrad chemistry courses? Is this a fairly new topic or is it just not important enough to be covered in classes? I ask because Normality kind of bugs me and most likely the MCAT will use Normality in titrations and solution chemistry.

          I doubt the MCAT will use normality as molarity is the common and unambiguous way to do it. Normality depends on the specific reaction you're talking about - molarity does not. So I wouldn't worry too much about it, but it's just a heads-up that you should have been taught this in gen chem because we normally cover it when we teach acids and bases.
           
          About the Ads

          betterfuture

          Full Member
          2+ Year Member
          Feb 16, 2016
          594
          96
            I doubt the MCAT will use normality as molarity is the common and unambiguous way to do it. Normality depends on the specific reaction you're talking about - molarity does not. So I wouldn't worry too much about it, but it's just a heads-up that you should have been taught this in gen chem because we normally cover it when we teach acids and bases.

            Thank you! I greatly appreciate your help!
             
            This thread is more than 5 years old.

            Your message may be considered spam for the following reasons:

            1. Your new thread title is very short, and likely is unhelpful.
            2. Your reply is very short and likely does not add anything to the thread.
            3. Your reply is very long and likely does not add anything to the thread.
            4. It is very likely that it does not need any further discussion and thus bumping it serves no purpose.
            5. Your message is mostly quotes or spoilers.
            6. Your reply has occurred very quickly after a previous reply and likely does not add anything to the thread.
            7. This thread is locked.