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Finding The Concentration Of An Ion

  1. Oct 27, 2015 #1
    • Please post this type of questions in HW section using the template.
    If I have a balanced chemical reactoin, for example

    KOH + HBr ==> H2O + KBr

    And I am given the concentration and volume of both KOH and HBr, how is it possible to determine the concentration of one of the ions from the net ionic equation?

    H + OH ==> H2O
  2. jcsd
  3. Oct 27, 2015 #2


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    Staff: Mentor

    In this particular case you will need to combine stoichiometry (hint: limiting reagent) with water autodissociation (water ion product).
  4. Oct 27, 2015 #3
    Sorry for placing this in the wrong thread.

    So basically find the limiting reagent between KOH And KBR, to determine the concentration of H2O from that, and then using H2O to find the concentration of OH?

    Then using the water autodissociation

    Kw = 1*10^(-14)mol^2/L^2 (assuming room temperature)

    where [OH][H] = Kw

    [H] = Kw/[OH]
  5. Oct 27, 2015 #4


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    Staff: Mentor

    No, concentration of H2O is (almost) constant. The only thing you need to check is the concentration of excess H+ or excess OH-. It may happen there is no excess of either - but then the problem is trivial.
  6. Oct 29, 2015 #5
    I'm going to restate the question with the given information, can you tell me if I am on the right track?

    In an experiment 0.5L of 0.020 mol/L of KOH is rapidly mixed with an equal volume of 0.020 mol/L of HBr. What is the initial concentration of H+.

    I Specifically need the concentration of [H+] at t = 0 to solve a 2nd order rate reaction. For the rate reaction i am given the constant K and the value of the concentration [H+] at time t.

    t = 1/k*(1/[H+]t - 1/[H+]0)


    KOH + HBr = > H2O + Kbr

    I am assuming that because KOH and HBr have the same concentrations and volumes that regardless of whether i use KOH or HBr to find the concentration of H2O i will get the same concentration.

    [H2O] = 0.02 MOL KOH / L KOH * 1Mol H2O/1MolKOH
    [H2O] = 0.02 MOL/L

    From the net ionic equation

    H+ + OH- = H2O

    [H+] = [H2O] = 0.02 MOL/L

    where this is the concentration at time t = 0
  7. Oct 29, 2015 #6


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    Homework Helper
    Gold Member

    There is misconception here and there is no meaningful experiment for such a reaction to be done. "KOH" is merely a manner of speaking, in a solution called e is only K+ and OH-. KBr is same thing and HBr pretty nearly. Reaction of H+ with OH- that happens when you mix these solutions is orders of magnitude faster that. You con follow by ordinary means.
  8. Oct 29, 2015 #7
    I don't entirely understand what you are saying in the post. What I am taking from what you said is when you mix KOH and KBr the reaction of OH and H happens as a result, which can be shown from the net ionic equation right? What I'm trying to find though, is an initial concentration of H+ so that i can determine how long it takes for the concentration of H+ to decrease to a certain amount. And I'm presuming I have to find the initial concentration of H+ using the concentrations of KOH and KBr?
  9. Oct 29, 2015 #8


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    Homework Helper
    Gold Member

    I am saying that when you mix these two things essentially all the chemical reaction that happens is the one you called net ionic reaction.
    This is the fastest chemical reaction known, instantaneous equilibrium for ordinary purposes. So only thing limiting rate of concentration change in any part of your reaction vessel is speed of mixing.
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