A gas occupied a volume of 3.50 L at 27 degrees celcius and 400.0 mmHg. What is the new temperature if the pressure changes to 0.789 atm and the volume is decreased to 2.50 mL
Use (P1V1/T1) = (P2V2/T2)
Don't forget T is in Kelvin.
You must change the volume so both are in the same unit (it makes no difference which one you change) and the pressure must be in the same unit (again, it makes no difference which one you change).