Choice of pressure when using ideal gas equation

jasoncurious

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?

Thanks for your help.

DrDu

Because initial and final pressure are equal as long as the weight on the cylinder remains the same.

Studiot

You can also use Charles Law to calculate the final volume, having calculated the initial volume from the universal gas law.