Stoichiometry Revision Notes for CAPS Physical Science Grade 12
Introduction
Stoichiometry is a branch of chemistry that deals with the quantitative relationships between the substances involved in chemical reactions. It allows you to calculate how much reactant is needed or how much product will be formed in a chemical reaction. This concept is crucial for understanding and executing various chemical reactions, which is vital for problemsolving in chemistry.
Key Points
 Mole Concept:
 The mole (mol) is the fundamental unit for measuring the amount of substance.

Avogadro’s number: (1 \, mol = 6.022 \times 10^{23}) particles.

Molar Mass:
 The mass of one mole of a substance (measured in g/mol).

Example: Molar mass of ( \text{H}_2\text{O} = 2 \times 1 \, \text{g/mol} + 16 \, \text{g/mol} = 18 \, \text{g/mol} )

Balanced Chemical Equations:
 Law of Conservation of Mass: Matter is neither created nor destroyed.
 Chemical equations must be balanced to reflect this law.

Example: ( 2 \text{H}_2(g) + \text{O}_2(g) \rightarrow 2 \text{H}_2\text{O}(g) )

Stoichiometric Coefficients:
 The numbers in front of molecules in a balanced equation indicate the ratio of moles of each substance.

Example: In (2 \text{H}_2(g) + \text{O}_2(g) \rightarrow 2 \text{H}_2\text{O}(g)), the ratio is 2:1:2.

Calculating Masses, Volumes, and Moles:
 Use the balanced equation to determine the relationships.

Equations:
 ( n = \frac{m}{M} ) (where ( n ) is moles, ( m ) is mass, and ( M ) is molar mass)
 ( c = \frac{n}{V} ) (where ( c ) is concentration, ( n ) is moles, ( V ) is volume)

Limiting Reagent:
 The reactant that is completely consumed in the reaction limits the amount of product formed.

The other reactant(s) will be in excess.

Percentage Yield:
 The actual amount of product obtained from a reaction expressed as a percentage of the theoretical maximum.
 ( \text{Percentage Yield} = \left(\frac{\text{Actual Yield}}{\text{Theoretical Yield}}\right) \times 100 \% )
RealWorld Applications
 Industrial Chemistry:
 Calculating the quantities of reactants needed to produce a desired amount of product.

Example: Production of ammonia via the Haber process.

Pharmaceutical Industry:
 Determining the correct doses of active ingredients in medicines.
Common Misconceptions and Errors
 Not Balancing Equations:
 Ensure all elements are balanced on both sides of the reaction.

Incorrect: (2 \text{H}_2 + \text{O}_2 \rightarrow \text{H}_2\text{O} )
Correct: ( 2 \text{H}_2 + \text{O}_2 \rightarrow 2 \text{H}_2\text{O} ) 
Confusing Moles and Molecules:

Remember that moles pertain to quantity in terms of Avogadro’s number.

Ignoring Limiting Reagent:
 Identify the limiting reagent correctly to predict the amount of product.
Practice and Review
 Practice Questions:
 Calculate the number of moles in 22 g of CO2.
 If 10 g of H2 reacts with O2, how many grams of H2O are formed?

Identify the limiting reagent in the reaction between 5 g of HCl and 10 g of NaOH.

Examination Tips:
 Carefully read the question for keywords like “limiting reagent” or “calculate the moles”.
 Always check that equations are balanced before performing calculations.
 Pay attention to units and ensure consistency throughout your calculations.
Connections and Extensions
 Interdisciplinary Links:
 Stoichiometry connects with biology in metabolic calculations.

It is also linked to environmental science in pollution control.

Extensions:
 Explore the use of stoichiometry in thermodynamic calculations in physics.
 Apply stoichiometric principles in laboratory settings to create solutions of precise molarity.
Summary and Quick Review
 Stoichiometry: Quantitative relationships in chemical reactions.
 Key Formulas: [ n = \frac{m}{M} ] , [ c = \frac{n}{V} ]
 Balanced Equations: Reflect the conservation of mass.
 Limiting Reagent: Determines the maximum product yield.
 Percentage Yield: Efficiency of a reaction.
Additional Resources
 Khan Academy: Comprehensive videos and practice problems.
 Chemguide: Detailed explanations and worked examples.
 PhET Interactive Simulations: Online labs and simulations to visualize reactions.
These notes provide a thorough yet accessible framework for understanding and mastering stoichiometry in Grade 12 Physical Science.