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normality vs molarity question/conundrum

Discussion in 'DAT Discussions' started by dentalstudent2021, Aug 4, 2015.

  1. #165 DAT destroyer.

    if 20 ml of .012 M solution of Ca(OH)2 is added to 48 ml of HBr, what is the concentration of the Hbr ?

    -- how do I do this and also I thought this was a morality question since its just asking for concentration. how do I differentiate a morality vs normality question.

    Thanks!
     
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  3. ThunderSplash

    2+ Year Member

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    I think you mean molarity. Anyways, The formula is still the same. M1V1=M2V2, where M is molarity and V is volume.

    In this case though, HBr gives 1 H+ but Ca(OH)2 gives 2 OH-. (If instead you had something like NaOH and HCl, you have a 1:1 ratio of H+ to OH- and you can just use M1V1=M2V2.)

    In this special case however, since the ratio of OH- to H+ is 2:1, you have to use the normality of the species. N1V1=N2V2. All you need to do then is since Ca(OH)2 gives off 2 OH-'s, you multiply its M (NOT VOLUME) by 2 to get it's N. This is just to account for it having more OH-'s than HBr has H+'s. Sorry if that is confusing, I just like to remember that if the ratio isn't 1:1, think about normality.

    So your equation becomes...

    N1V1=N2V2
    (.012M x 2)( .02L)=(N2)(.048L)
    (.024M)(.02L)=(N2)(.048L)
    (.00048 MxL)=(N2)(.048L)
    (.00048 M x L)/(.048L)= N2
    N2= 0.01M (since HBr only gives 1 H+, you don't need to adjust for it's normality)
     
    #2 ThunderSplash, Aug 4, 2015
    Last edited: Aug 4, 2015
    jjj7236 likes this.

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