Phosphorus pentachloride is formed when 17.2 g of chlorine gas react with 23.2 g of solid phosphorus (P2). Determine the reactant that is in excess.

The balanced equation for the above reaction is;

P₂ + 5Cl₂ - - - > 2PCl₅

Stoichiometry of P₂ to Cl₂ is 1:5

Number of P₂ moles present - 23.2 g / 62 g/mol = 0.37 mol

Number of Cl₂ moles present - 17.2 g / 71 g/mol = 0.24 mol

If Cl₂ is the limiting reactant,

then number of P₂ moles that react with 0.24 mol of Cl₂ - 0.24/5 = 0.048 mol

Only 0.048 mol of P₂ is needed, but 0.37 mol of P₂ is provided, therefore P₂ reactant is in excess.

Excess reactant is the reagent provided in excess and only a fraction of this amount is used up for the reaction.

An amount of 0.37 - 0.048 = 0.322 mol of P₂ is remaining after the reaction