Question

P4(s)+10Cl2(g)--4PCl5(g)
Now consider a situation in which 21.0 g of P4 is added to 60.0 g of Cl2, and a chemical reaction occurs. To identify the limiting reactant, you will need to perform two separate calculations:
Calculate the number of moles of PCl5 that can be produced from 21.0 g of P4 (and excess Cl2).
Calculate the number of moles of PCl5 that can be produced from 60.0 g of Cl2 (and excess P4).
Then, compare the two values. The reactant that produces the smaller amount of product is the limiting reactant.