To
determine the number of moles of gas, we can use the ideal gas law equation: PV = nRT. Given that the pressure (P) is 1.2 atm, the volume (V) is 31 liters, and the temperature (T) is 87°C, we need to convert the temperature to Kelvin (K) by adding 273.15. Then we can solve for the number of moles (n).