Question: how many oxygen molecules are needed to synthesize 0.245g of carbon monoxide?
Answer: 1 mole of oxygen reacts with 2 moles of carbon to produce 2 moles of CO. Now 0.245g of CO = 0.245/28 = 0.0088 mole and this will require 0.0088/2 = 0.0044 mole of oxygen. Every mole of oxygen contains 6.022 x 10^23 molecules of oxygen.