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Author: Subject: Simple molar concentration problem
SunriseSunset
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mad.gif posted on 17-6-2015 at 20:22
Simple molar concentration problem


Ok fellow amateur scientists of our society.. I have a question that needs answers once again.. This simple concept is boggling my existence. Seriously! Let me begin by saying that I've accomplished fair numbers decent synthesis problems on my own, with alright results prior over some years

and suddenly I've run into a basic equation that denies almost all of the logic that I have used up to now.. So understandably, I am pretty dumbfounded.

What volume of 12.0 M HCl is required to make 75.0 mL of 3.50 M HCl?

question seems pretty straight forward. From all of my understanding, this means 12 M means Molar which means moles per liter. (ffs) lol

And yet when I find out the equation people use is 12.0V1 = 3.5*75.0

solve for V1, and you get: 21.875ml it pisses me off so hard because that makes no sense if 12.0 M HCl represents 12 moles per LITER... Haha PLEASE shine some LIGHT!

To me, it only makes sense that 291.67ml of a 12 M HCl solution would enough to provide 3.5 moles of HCl. And obviously 291.67ml is far more than the 75ml it asks for. Have I been lied to, or am I just INSANE?!

[Edited on 18-6-2015 by SunriseSunset]




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j_sum1
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[*] posted on 17-6-2015 at 20:49


Quote: Originally posted by SunriseSunset  
To me, it only makes sense that 291.67ml of a 12 M HCl solution would enough to provide 3.5 moles of HCl. And obviously 291.67ml is far more than the 75ml it asks for. Have I been lied to, or am I just INSANE?!

Well, I have no idea how you got that figure of 291.67mL
Care to enlighten?
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Fulmen
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[*] posted on 17-6-2015 at 22:31


22ml is correct. As you've already know, M means molar which is moles per liter. So in order to make 75ml of 3.5M HCl solution you need 0,075liters*3,5moles/liter=0,2625moles of HCl diluted to 75ml. Since your stock is 12M you need 0,2625moles/12moles/liter=0,021875liters.



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DraconicAcid
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[*] posted on 17-6-2015 at 22:38


Quote: Originally posted by SunriseSunset  

To me, it only makes sense that 291.67ml of a 12 M HCl solution would enough to provide 3.5 moles of HCl. And obviously 291.67ml is far more than the 75ml it asks for. Have I been lied to, or am I just INSANE?!


But you don't want 3.5 mol of HCl. You want 75 mL of a 3.5 mol/L solution, which is 7.5 thousandths of 3.5 mol.




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Pyro
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[*] posted on 18-6-2015 at 01:44


no, 75 thousandths of 3.5 mol.
291.67ml is enough for 1l of 3.5M
if you want 0.075l you need 21.9ml of 12M and then add water until you get to 75ml

you got a strange result because you mixed used ml instead of l

Try this:
12M*V1=3.5M*0.075l
V1=(3.5M*0.075l)/12M
V1=0.021875l=21.875ml

[Edited on 18-6-2015 by Pyro]




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DJF90
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[*] posted on 18-6-2015 at 01:54


Using the equation C1V1 =C2V2, it does not mater what units are used so long as they are the same on both sides, e.g. concentration can be expressed as M, %w/w etc, and volume can be mL or L (or any other unit of volume!).

You got the right result, its just that your interpretation that was faulty (i.e. moles is not the same as molarity (unless considering a volume of 1 L)
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Pyro
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[*] posted on 18-6-2015 at 02:08


upon closer examination you appear correct :)
He misinterpreted the result...

It is good practice to always use l though




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SunriseSunset
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thumbup.gif posted on 18-6-2015 at 22:47


Oh it makes sense now. Wow. Thanks for breaking that down too. damn...



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