## Choice of pressure when using ideal gas equation

100g of CO are contained in a weighted piston-cylinder device. Initially, the CO is at 1000kPa and 200°C. It is then heated until 500°C. Determine the final volume of the CO treating it as an ideal gas.

This is what my lecturer gave in his solution:

V2=(mRT2)/P=(0.1kg*0.2968kPa.m3/kg.K*(500+273)K)/1000kPA

My question is: why is the original pressure (1000kPa) been used instead of the final pressure?