In a chemical reaction. How do we know which of the two substance is IN EXCESS?

Question

callisto · Answer

determine the number of mole of each reactant, the smallest one is the limiting agent, while the other reactants are in excess

callisto · Answer

eg, 
10g of MgO reacts with 100cm^3 of 0.1M HCl
MgO + 2HCl -> MgCl2 +H2O
number of mole of MgO used = 10/(24.3+16.0) = 0.025720mole
number of mole of HCl used = 100/1000 x 0.1 = 0.01 mole
since 1mole of MgO required  2 mole of HCl for complete reaction.
0.025720mole of MgO needs 2x0.025720mole = 0.05144 mole of HCl for complete reaction
However we have only 0.01mole of HCl for the reaction, which is not enough. Therefore, HCl is the limiting agent. On the contrary, MgO is in excess.

anonymous · Answer

you know that nitrogen and hydrogen reacts in ratio 1 to 3. suppose nitogen given is 1 mole and hydrogen given is 2 mole. clearly hydrogen is less....so to find limitung reagent , you have to consider the stoicometric ratios and use it to find which is in excess, is it clear now?