When to use 0.0821 vs 8.21 for R

[joshto]

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I think I came over a few questions, both using R in the answer formula.

sometimes 0.0821 was used, other times 8.21 was used.

Any clarification on when to use which?

 

[Geekchick921]

8.31

It depends on the units for the rest of the problem, you want them to match.

If pressure is given in pascals and volume is given in m^3, use 8.31 J/mol*K, as a pascal (which is a N/m^2) and m^3 multiplied together give you Joules (Newton * m).

If the pressure is given in atmospheres and volume is given in liters, use .0821 L*atm/mol*K.

If you're given something crazy like pascals and liters or atmospheres and m^3, convert one to the other so you can use 8.31 or 0.0821. A cubic meter is 1000L, and an atmosphere is about 100kPa (101.325kPA precisely).

 

[Flapjacks]

Normally when you use .08206 it will be at standard condition as well (298 K) so remember that calculates to 22.41.

Also, 8.314 would be (not limited to) heat capacity/ heat equations.