Limiting Reagents

In a chemical reaction, one of the reactants is often not completely consumed. This reactant is known as the the "excess". The reactant is completely consumed is called the limiting reagent.

Copper and elemental sulfur react to produce copper (I) sulfide. If you have 10.0g of copper and 10.0 g of sulfur, determine the limiting reagent.

solution
Equation:

Convert grams to moles:

Since , copper is the limiting reagent

From the previous example, determine how many moles of the excess reagent is left over "unreacted"

solution
Using Stoichiometry, determine how much is used:

Subtract from how much was used (0.312 from previous calculation)

So 0.234 moles of sulfur leftover.

Zinc is reacted with hydrochloric acid to produce hydrogen gas and zinc chloride. If 10.0g of zinc was reacted with 0.50mol hydrochloric acid, calculate the number of moles of the excess reagent left over.

solution
Equation:

Moles:

Since , zinc is the limiting reagent.

So 0.19 moles of excess HCl.