50 cm3 of carbon (II) oxide was exploded with 150 cm3 of air containing 20% oxygen by volume. Which of the reactants was in excess?
Carbon (II) oxide
Carbon (IV) oxide
Oxygen
Nitrogen
Explanation
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SOLUTION
carbon(ii)oxide = 50cm
oxygen = 150cm
percentage 20/100
solution for carbon(ii)oxide
50/1 * 20/100 =10cm
solution for oxygen
150/1 *20/100 = 30cm
and the question says by volume which reactant is excess. from the solution 30cm is higher which is oxygen. so the correct ans is c
20/100×150=30cm³of O2
2CO+O2 --------- 2CO2
2mole of CO ------- 1mole of O2
50cm³ --------- 50/2=25cm³
30cm³ was available for the reaction but only 25cm³ is required :. O2 is in excess
So the thing is,they said 50cm3 of CO was exploded with 150cm3 of air and in that air we have 20% oxygen so if we take the 20% oxygen from the air we have 30cm3 of oxygen.
CO+O2=CO2
if we balance the equation we have
2CO+O2=2CO2
remember we were given 50cm3 of CO and 30cm3 of oxygen and if you remember your Gay-lussac's law CO and O are in the ratio of 2:1 so if we have 50cm3 of CO what next of course we will need only 25cm3 of oxygen for the reaction according to Gay-lussac. So since only 25cm3 of oxygen was used there is still 5cm3 of unused oxygen. Hence oxygen is in excess by 5cm3 because not all the oxygen was used up.I just hope this helps.
in fact I've it very good 
with the knowledge of stoichiometry, oxygen is a limiting agent, in another word is called short supply. oxygen is a limiting agent because 30cm where needed for the reaction, but only 25cm take part in the reaction. also remember that in a reaction, limiting agent is the one that determine the product of the reaction, have that in mind. so with the few back up points, is clear that oxygen is not in excess, thanks
