In a closed piston-cylinder system, methane was observed to obey the following equation of state P (V - nb) = nRT where b = 0.029 m³/mol. The temperature and volume are 500 ºC and 5 m³ respectively for 100 moles of methane. At this state of the system, the isobaric rate of change of temperature with volume (in ºC/m³) is __________ (rounded off to second decimal place).