Stoichiometry deals with ratios of reactants to products. When we have a 3:1 ratio (e.g. K3PO4) we are dealing in moles rather than in mass.

Limiting Reagent vs Excess Reagent

If the provided moles of reagents are not in the correct ratio (e.g. exactly 3:1 in the above example), then not all of the reactant will be used.

The reagent with more moles (in ratio) than the other is known as the 'excess reagent', and will have an unreacted portion left over (the number of moles extra).

The reagent with less moles is known as the 'limiting reagent', because it is its mass that determines the end of the reacting.

The maximum moles of product produced is determined by the limiting reagent.