When working with gas laws, the temperature must always be in Kelvin. To calculate this, simply add 273 to the temperature in Celsius. To find the gas constant assume you are under STP conditions.
To find the gas constant, assume you are at Standard Temperature and Pressure conditions (STP) and you have one mole of gas.
Then, P= 1 atmosphere, T= 273 K, V= 22.4liters and n= 1 mole.
When you plug these values into the ideal gas law formula:
PV=nRT, you find that R, the constant, is equal to 0.0821. This is the constant as long as you remember to change your pressure to atmospheres (mm Hg/760 = atm), your volume to liters (ml/1000 = liters), and of course your temperature must always be in Kelvin (C + 273).
For your problem:
(1.95 atm) (12.30) = (.654 moles) (0.0821) (K)
You can't solve one equation in two unknowns.
The value of the gas constant
In this problem, you have to find
The temperature of the gas is 174 °C.