# How do you know which ideal gas constant to use?

##### 1 Answer

Mar 14, 2014

The ideal gas constant that you will use will depend on the units of the known quantities in the problem.

#### Explanation:

You will choose the R value based off of the units for the known quantities in the problem.

You will have values or be looking for values for:

V - usually in liters

T - Kelvin (convert to Kelvin if given Celsius or Fahrenheit)

n = moles

P = Pressure (atm, mmHg, Torr, kPa...)

The key is usually pressure.

For P in atm use R = 0.082057 atm*L/mol*K

For P in kPa use R = 8.31446 kPa*L/mol*

For P in mmHg or Torr use R = 62.36367 mmHg*L/mol*K

See the similarities in all of these? Just the pressure is different.

If the problem you are working on provides different units for the quantities, you can look up other R values here