Question

When to use which value of gas constant

Answer 1

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 atmL/molK
For P in kPa use R = 8.31446 kPaL/mol
For P in mmHg or Torr use R = 62.36367 mmHgL/molK