# TPR SWB G-Chem Free Stand No.55

Discussion in 'MCAT Study Question Q&A' started by HealthHare, Aug 29, 2013.

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1. ### HealthHare 5+ Year Member

374
38
Jul 17, 2010
USA
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

869
271
Feb 11, 2013
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

374
38
Jul 17, 2010
USA
Thank you very much, this helps!