Stoichiometry

1. Calculate the change in amount (moles, mass, or volume) of a reactant or product in a chemical reaction
2. Identify limiting and excess reagents, and the theoretical and actual yields of products
3. Calculate % yield

Finding Limiting and Excess Reagents

Limiting Reagent: The reactant that is completely consumed in a chemical reaction. The amount of product formed is limited by this reactant.

Excess Reagent: The reactant that is not completely consumed in a chemical reaction. There is more of this reactant than is needed to react with the limiting reagent.

To find the limiting reagent, calculate the amount of product that can be formed from each reactant. The reactant that produces the least amount of product is the limiting reagent. For example, if and react to form and , and you are given that there is only 6.00 g of aluminum and 16.0 g of rust:

Balanced equation:

Because , is the limiting reagent, making Al the excess reagent.

Theoretical Yield

In the previous problem, knowing now that all 0.100 mol of will react, we can calculate the theoretical yield of by drawing a BCA (Before, Change, After) table:

Balanced equation:

Before	0.100 mol	0.222 mol	0	0
Change	-0.100 mol	-0.200 mol	0.200 mol	0.100 mol
After	0	0.022 mol	0.200 mol	0.100 mol

Tip

To figure out how many moles of products are formed, look at the coefficients in the balanced equation. In this case, 2 moles of are formed for every 1 mole of that reacts, so Fe’s theoretical yield is 0.200 mol. Similarly, 0.100 mol of is formed because ‘s coefficient has a ratio of 1 with .

% Yield

Percent Yield: