Stoichiometry: Chemical Quantities and Calculations

Introduction to Stoichiometry

Stoichiometry is the branch of chemistry that deals with the quantitative relationships between reactants and products in a chemical reaction. It allows chemists to predict the amounts of substances consumed and produced in a given reaction.

• Stoichiometric calculations are based on balanced chemical equations.

• Key concepts include mole, molar mass, Avogadro's number, and percent composition.

• Stoichiometry is essential for laboratory work, industrial processes, and understanding chemical reactions.

Molecular and Formula Weights

The molecular weight (or molar mass) of a compound is the sum of the atomic masses of all atoms in its chemical formula.

• Molecular weight is measured in atomic mass units (amu) or grams per mole (g/mol).

• To calculate, add the atomic masses of each element multiplied by the number of atoms present.

• Formula weight applies to ionic compounds, while molecular weight applies to covalent compounds.

Example: The formula weight of NaCl is calculated as:

Percent Composition

Percent composition expresses the mass percentage of each element in a compound.

• Formula:

• Used to determine the empirical formula and analyze purity.

Example: For :

• Calculate mass of each element in 1 mol.

• Divide by total molar mass and multiply by 100%.

The Mole Concept and Avogadro's Number

A mole is a unit that represents particles (atoms, molecules, ions).

• Avogadro's number is .

• 1 mole of any substance contains Avogadro's number of particles.

• Used to convert between mass, moles, and number of particles.

Example: mol of Cu contains: atoms of Cu

Stoichiometric Calculations: Mole-to-Mole and Mass-to-Mass

Stoichiometric calculations use balanced equations to relate moles and masses of reactants and products.

• Steps: (1) Write balanced equation, (2) Convert mass to moles, (3) Use mole ratio, (4) Convert moles to mass.

• Mole ratios are derived from coefficients in the balanced equation.

Example: For :

• Given mass of NaO$_2$, calculate moles.

• Use mole ratio to find moles of O produced.

• Convert moles of O to grams.

Limiting Reactant and Theoretical Yield

The limiting reactant is the substance that is completely consumed first, limiting the amount of product formed.

• Identify limiting reactant by comparing mole ratios.

• Theoretical yield is the maximum amount of product possible.

• Actual yield is often less due to losses; percent yield is calculated as:

Example: If 20 g of SO is produced from a reaction with a theoretical yield of 28.04 g:

Worked Examples and Practice Problems

• Calculating number of atoms in a given mass.

• Finding mass of product from a given mass of reactant.

• Determining limiting reactant and percent yield.

Example: How many moles of oxygen are in 0.12 mol of Al(NO)$_3$?

• Each mole of Al(NO)$_3$ contains 9 moles of O.

• mol O

Summary Table: Key Stoichiometric Relationships

Additional info:

• Some equations and examples were inferred for clarity and completeness.

• Images included only where they directly reinforce calculations or concepts.