The molar volume of helium is 44.8 L at :

V = n R T 

P PV=nRT

 we have n=1 mole V=44.8L 

equation become 44.8P=RT 

if we give a value for T we get value for P corresponding to that T If we give a value for P we get value of T at that P ie, if we assume temperature value ,we get the value of pressure of 1 mole of He at the assumed temperature. Similarly if assume pressure, we will get the value of temperature for assumed pressure If we choose T = 273 k we get P = ( R × 273 ) / 44 . 8 = 50 . 67 a t m atm