What mass of co was used up in the reaction with an excess of oxygen gas if 24.7g of carbon dioxide is formed? 2 co + o2 > 2 co2

Balance Chemical Equation,
                                      2 CO  +  O₂   →   2CO₂
Acc. to this reaction,
88 g (2 mole) of CO₂ was produced when  =  56 g (2 mole)of CO was reacted
So, 
24.7 g of CO₂ will be produced by reacting =  X g of CO

Solving for X,
                                    X  =  (56 g × 24.7 g) ÷ 88 g

                                    X  =  2.26 g ÷ 88 g

                                    X  =  0.0257 g of CO

Result:           
            0.0257 g of CO is required to be reacted with excess of O₂ to produce 24.7 g of CO₂.