1. Unit 4: Chemical Equations and Stoichiometry
Stoichiometry and Limiting Reagents

2. Stoichiometry
Stoichiometry is a fancy chemistry word meaning “to calculate unknown quantities in a chemical reaction”.
Or in other words: if I mix this much of “X” and this much of “Y” they will react to form this much of “Z”.
Remember: the coefficients in the balanced equation are mole ratios, NOT mass ratios (this means you have to get things into moles first)

3. Example #1
Lithium hydroxide reacts with gaseous carbon dioxide to form solid lithium carbonate and liquid water. How many grams of lithium carbonate are produced from 1.00 g of lithium hydroxide?
Step 1: Write and balance the chemical equation
Step 2: Convert grams → moles (use molar mass)
Step 3: Convert known moles to unknown moles using the balanced chemical equation
Step 4: Convert moles → grams using the molar mass

4. Example #1 Work

5. Practice Problem #1
How many grams of hydrogen are produced when g of hydrochloric acid (HCl) are reacted with copper?
HCl + Cu  CuCl2 + H2

6. Limiting Reactants
Limiting reactant: the reactant that is completely consumed in a reaction and limits the amount of product formed

7. Limiting Reactants Example #1
A strip of zinc metal with a mass of 2.00 g is placed in an aqueous solution containing 2.50 g of silver nitrate. How many grams of silver will form?
Step 1: Write and balance the chemical equation
Step 2: Convert grams → moles (use molar mass)
Step 3: Convert known moles to unknown moles using the balanced chemical equation
Step 4: Convert moles → grams (use molar mass)
Step 5: Do steps 2-4 for as many reactant amounts as you are given to determine the limiting reactant
- choose the smallest amount!

8. Limiting Reactants Example #1 Work

9. Limiting Reactants Practice #1
If 120. g of sodium carbonate react with 105 g of calcium hydroxide, how many grams of sodium hydroxide are formed?
Na2CO Ca(OH)2  NaOH CaCO3

10. Molar Volume of a Gas
At standard temp and pressure (STP) 1 mole of any gas occupies 22.4 L
1 mole = 22.4 L (use this as a conversion factor)
Example:
How many liters will 48.6 grams of CO2 occupy at STP?

11. H2S (g) + O2 (g) → H2O (l) + S (s)
Gas Stoich Example #1
In the following reaction at STP, 3.25 g of solid sulfur was collected. Calculate the volume of H2S gas used.
H2S (g) + O2 (g) → H2O (l) + S (s)

13. Gas Stoich Example #2
An excess of hydrogen gas reacts with 14.0 g of nitrogen gas to produce ammonia (NH3). How many liters of ammonia are produced at STP?