Miranda A.

asked • 06/28/15

What is the new concentration when 78.2 mL of a 15.0 M solution of HBr was diluted to 988 mL?

0.908 is the answer. Just need a how to do it. Thanks!

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Harvey F. answered • 06/29/15

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Hi Miranda,
Convert 78.2mL to .0782L then multiply by the molarity to get the number of moles of HBr.
That's .0782L x 15.0mol/L = 1.173 mol of HBr.
After dilution you have a volume of .988L so divide the moles by the volume to get the new molarity.
That's 1.173 mol/.988L = 1.19M of HBr solution.
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JOHN F. answered • 06/28/15

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JOHN F.

Miranda,
I'm a little confused with the units provided.

I know that mL = milliliters, but what is M? I'm not familiar with a volume measurement of M, just a length measurement (m = meters).
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06/28/15

Miranda A.

Moles per liter
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06/28/15

JOHN F.

Never mind, I found out that it's the molarity (concentration) of the solution.  I'll see what I can figure out now that I know that.
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06/28/15

JOHN F.

Miranda,
I've worked it two different ways and both ways I get 0.8422, not 0.908, so I obviously don't know what I'm doing on this problem.  I'm sorry I couldn't help.
John
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06/29/15

Miranda A.

That's alright. Thank you for attemping it 
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06/29/15

Harvey F.

John, I noticed that your answer is the reciprocal of my answer. Could you share how you worked the problem with me so we can come to a consensus?
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06/29/15

JOHN F.

Harvey, I am away from home (and my notes).  Our answers, being what they are (reciprocals of each other, so I believe they support each other and I have my doubts about 0.908 being the answer), are the same in a way, I suppose.  60mi/hr and 1hr/60mi are the same, are they not?  The way your answer is annotated projects confidence and I'd go with your answer over mine.
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07/07/15

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