Question

Chemistry

A gas occupies 250 mL and its pressure is 550 mmHg at 25 C.

I used R = 6.24 x 10^4
I converted the ml into L = 0.250
I got n = (550(0.25)/(6.24)(298) =7.4 x 10^-6
However, the book converted the mmHg to atm and used the R = 0.0821.
They got n = (0.724 atm)(0.25)/(0.0821)(298K)
= 7.4 x 10^-3
How come they got a different answer? How come I got a different answers although I used the right units for R? Does these calculations always have to be made in atm?

ALso, what are the conversion factors between atm, Joule, mmHg, calories, and volt coulomb so I can use the R units.
Thank you.

Answer 1

You can calculate the answer using pressure in any unit, you just have to make sure your conversion of R is correct. If you choose to use mmHg, R is 6.24*10 (62.4). Also in your equation above, you used 6.24. Using 62.4, you will get 7.4*10^(-3) moles.

1 atm = 760 mmHg = 760 Torr = 101.3 kPa --> units of pressure

1 L*atm (that's litre atmospheres) = 101.3 J = 24.2 cal --> units of energy
A more useful conversion might be 1 cal = 4.18 J = 4.18 V*C (that's volt coulombs) --> units of energy

Hope that helps!

Answer 2

Thanks, I think there's an error in my R value table.