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August 28, 2014

August 28, 2014

Posted by **terrence** on Wednesday, August 31, 2011 at 6:51am.

In the question: Calculate the amount of moles of a gas if .455 occupies a volume of 250mL at 27 degrees Celsius and 772mmHg.

I do all the correct steps using the R = 8.314 for the 772mmHg. I get .077 for this, which is incorrect.

Then I do the same thing but convert the 772mmHg to 1.016atm and for the gas constant use .08206. I get .0103 which is the correct answer. Why does it make a difference which I used? In fact i tried using the larger constant with units of Pa in a question and didnt get it right, but did it again with the smaller constant in units atm and got it right. Please please help this confusion. Thanks

- chemistry -
**DrBob222**, Wednesday, August 31, 2011 at 7:09pmR is 0.08206 with V in L and P in atm.

R is 8.314 with V in L and P in kPa.

By the way, I have no idea what the 0.455 is in your post and there are no units. I ignored it.

0.0103 is n either way.

n = (PV/RT) = (772 x 0.250)/(760 x 0.08206 x 300) = 0.0103 = n

772 x 101.325/760 = 102.92 kPa.

(102.92 x 0.250)/(8.314 x 300) = 0103 = n

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