When to use 0.0821 vs 8.21 for R

joshto

Nervous&Neurotic
10+ Year Member
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

Achievement Unlocked: MD
Moderator Emeritus
10+ Year Member
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

Membership Revoked
Removed
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.