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TPR SWB G-Chem Free Stand No.55

Discussion in 'MCAT Study Question Q&A' started by HealthHare, 08.29.13.

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  1. HealthHare

    HealthHare 5+ Year Member

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    The resulting solution made from the combination of 50ml of 1.0M LiOH with 50ml of 1.0M HBr will be identical in all respects to 100ml of:

    A)0.5M LiBr
    B)1.0M LiBr
    C)2.0M LiBr
    D) a saturated solution of LiBr (s)

    Picked B but the answer was A. I reasoned that

    (MV)LiOH +(MV)HBr = (MV)LiBr so 1(50) + 1(50) = (100)M Libr so M = 1M LiBr

    I don't understand why the answer is .5M LiBr, can someone please explain?
     
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  3. csdanim

    csdanim 2+ Year Member

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    To make LiBR, you need the same moles of LiOH and HBr. First find how many moles of each are in the solution. V*M gives 50mmol of LiOH and HBR. that also means there are 50mmols of Li+,OH-,H+,Br-. Li+ and Br combines to give 50mmol of LiBr in total volume of 100ml. Molarity of LiBr is thus 50mmol/100ml = 0.5M

    Edit:Your reasoning is incorrect because you're thinking 50mmol of Li+ and 50mmol of Br- will give 100mmol of LiBr.
     
  4. HealthHare

    HealthHare 5+ Year Member

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    Thank you very much, this helps!
     

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