b) A 40 g of oxygen gas, Oz in a cylinder of volume 1.5 x 102 m' has a temperature of 350 K. The number of moles of oxygen gas present initially in the cylinder is 1.10 mol. After the gas has been used at a constant temperature, the pressure reduces to 1.2 x 105 N m-2. Estimate the number of moles of gas left. (4 marks)