We use stoichiometry when given the KNOWN quantity of a compound or element and asked to find the UNKNOWN quantity of another compound or element in a balanced chemical equation. 

Stoichiometric Reactions

Example: How many grams of H2O are produced when 12.3 g Hreacts?


Remember whenever you have to find the unknown quantity of a compound or element just start with your given quantity and use the stoichiometric chart for guidance. 

Stoichiometric Reaction Calculations

The word “excess” means that a compound isn’t important in our calculations and can be ignored. 

Example: The oxidation of chromium solid is represented by the following equation:  

a)   How many moles of chromium (III) oxide are produced when 34.69 g Cr reacts with excess oxygen gas?

b)   How many grams of O2 were needed to produce 4.28 x 103 molecules Cr2O3


What happens when we mix stoichiometry and the old concept of density? Let’s see. 

Example: If the density of ethanol, CH3CH2OH, is 0.789 g/mL, how many milliliters of ethanol are needed to produce 4.8 g of H2O in the following reaction? 


For those up for a challenge you maybe given the compounds in a reaction and asked to write it out and balance it, followed by doing the stoichiometric problem. 

Problem: Dinitrogen monoxide gas decomposes to form nitrogen gas and oxygen gas. How many molecules of oxygen are formed when 8.00 g of dinitrogen monoxide decomposes?


Limiting Reactant or Reagent

The limiting reactant or limiting reagent is the compound that determines the maximum amount of product obtained in a chemical reaction. 

Concept: Understanding the limiting reactant or reagent


In a limiting reagent or reactant question we are given information on more than one compound and asked to find the unknown quantity of another compound (usually the product). 

Example: Chromium (III) oxide reacts with hydrogen sulfide (H2S) gas to form chromium (III) sulfide and water:

What is the mass of chromium (III) sulfide formed when 14.20 g Cr2O3 reacts with 12.80 g H2S? 


The limiting reactant is the one that shows a smaller product yield, with that yield representing the theoretical yield. The excess reactant is that one that shows a greater product yield.

Example: Identify the limiting reactant, excess reactant and theoretical yield


Percent Yield

The percent yield determines how efficiently your reaction was in producing products. The higher the percent yield then the more successful the chemical reaction

The amount of product we isolate when a lab experiment is done is called the actual yield. Relating that with our theoretical yield helps us determine the percent yield of our reaction. 

Example: A scientist performs an experiment in the laboratory and obtains 13.27 g Cr2S3. If his calculations on scratch paper give him a theoretical yield of 18.23 g what is the percent yield?


By combining your knowledge of stoichiometry to find the theoretical yield we can now use the actual yield to find the percent yield of a chemical reaction. 

Problem: Consider the following balanced chemical reaction: 2 C6H6 (l) + 15 O2 (g) ----> 12 CO2 (g) + 6 H2O (l) a. If a 2.6 g sample of C6H6 reacted with excess O2 to produce 1.25 g of water, what is the percent yield of water?


Problem: Consider the following balanced chemical reaction: 2 C6H6 (l) + 15 O2 (g) ----> 12 CO2 (g) + 6 H2O (l) b. If the above reaction only went to 75% completion, how many moles of CO2 would be produced if 1.57 x 10-5 molecules of C6H6 were reacted with excess oxygen?