Question

What volume of oxygen gas at NTP is necessary for complete combustion of 20 litre of propane measured at 0°C and 760 mm pressure. 

Answer 1

The balanced equation is 

1 litre of propane requires = 5 litre of oxygen 

20 litre of propane will require = 5 x 20 = 100 litre of oxygen at 760 mm pressure and 0 °C.