Chemistry / Unit 3 Chem 0480 worked example, 60 to 90 seconds
Determining the Limiting Reactant
The idea
Determining the limiting reactant means calculating the amount of one product each reactant could form entirely on its own, then identifying whichever reactant yields the smaller amount of that product.
In practice
For 2.00 g of Si reacting with 1.50 g of N2 to form Si3N4, silicon alone, converted through moles and the balanced equation's ratio, could form about 3.33 g of Si3N4. Nitrogen alone, converted the same way, could form about 3.76 g of Si3N4. Silicon yields the smaller amount of product, so silicon is the limiting reactant and nitrogen is present in excess, left over once the reaction stops.
Where people slip
Comparing the two reactants' starting masses directly, 2.00 g against 1.50 g, and picking the smaller one skips the mole-ratio conversion this determination actually requires.
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