If 23.96 kg of iron (III) oxide are combined with 11,200 dm3 of carbon monoxide
at STP, which of the following substances would be the excess reactant?
Fe2O3 (cr) + 3CO (g) ---> 2Fe (cr) + 3CO2 (g)

Hi teaetea!!

At STP one mol of a gas has a volume of 22.4 L or dm3, then at STP
11.2 L are occupied by 0.5 mol of gas; in this case the gas is CO.
For this reaction the minimum amount of Fe2O3 required for a complete
reaction is 1/3*0.5 = 1/6 mol of Fe2O3.
The molecular mass of the Fe2O3 is: 2*55.85g + 3*16g = 159.7g , and
this is the weight in grams of one mol of Fe2O3.
Then 1/6 mol of Fe2O3 will weight 1/6*159.7g = 26.62g and this is the
minimum amount of Fe2O3 in grams required for a complete reaction.
We have 23.96g of Fe2O3 and this amount of Fe2O3 is less than the
minimum required of 26.62g.
The conclusion of this is that the Fe2O3 is the limiting reactant,
then the CO is the excess reactant.

See the following page for more help:
"Stoichiometry :  Limiting Reactant":
http://www.chem.tamu.edu/class/majors/tutorialnotefiles/limiting.htm

I hope this helps.

Regards.
livioflores-ga