# 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 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?

3. ### 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 5+ Year Member

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372
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Thank you very much, this helps!