Posted by terrence on Wednesday, August 31, 2011 at 6:51am.
I'm having a problem with the gas constant. This is it:
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 didn't 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 7:09pm
R 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