Stoichiometry in Chemical Reactions

Introduction to Stoichiometry

Stoichiometry is the quantitative study of reactants and products in chemical reactions. It allows chemists to predict the amounts of substances consumed and produced, based on balanced chemical equations.

• Stoichiometry involves using mole ratios from balanced equations to calculate quantities of reactants and products.

• Always convert masses or volumes to moles before performing stoichiometric calculations.

• Units such as grams, liters, or molecules must be converted to moles for accurate calculations.

Example: For the reaction: 2 moles of potassium chlorate yield 2 moles of potassium chloride and 3 moles of oxygen gas.

Using Mole Ratios

Mole ratios from balanced equations are used to relate quantities of different substances.

• For every 2 moles of KClO3, 3 moles of O2 are produced.

• To find the amount of product formed, use the mole ratio and the amount of reactant available.

Example: If you have 20 ounces of tomato sauce and enough of all other ingredients, you can make 4 pizzas, based on the ratio in the recipe analogy.

Limiting Reactants and Theoretical Yield

Identifying the Limiting Reactant

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

• Compare the mole ratios of reactants to determine which will run out first.

• The amount of product formed is determined by the limiting reactant.

Example: In the synthesis of ammonia: If you have 5.22 kg of H2 and 31.5 kg of N2, calculate which is limiting and the theoretical yield of NH3.

Theoretical Yield, Actual Yield, and Percent Yield

The theoretical yield is the maximum amount of product that can be formed from the limiting reactant. The actual yield is the amount actually obtained from the reaction. Percent yield measures the efficiency of a reaction.

• Percent yield formula:

• Actual yield is often less than theoretical yield due to side reactions, incomplete reactions, or loss of product.

Example: For the reaction: If 85.8 g carbon monoxide produces 72.3 g iron, calculate the limiting reactant, theoretical yield, and percent yield.

Solution Concentrations

Molarity and Dilution of Solutions

Molarity (M) is defined as moles of solute per liter of solution. Dilution involves adding solvent to decrease the concentration of a solution.

• Molarity formula:

• Dilution formula:

Example: To dilute 52.10 mL of 0.178 M HCl to make 0.132 M solution: mL

Other Units for Solution Concentrations

Besides molarity, solution concentrations can be expressed in mass percentage, volume percentage, mass-volume percentage, parts per million (ppm), and parts per billion (ppb).

• Mass percentage:

• Volume percentage:

• Mass-volume percentage:

• Parts per million (ppm):

• Parts per billion (ppb):

Example: If a 200.0 g sample of solution has 390 mg of sodium ion, the concentration in ppm is: ppm

Tables

Summary Table: Solution Concentration Units

Key Definitions

• Stoichiometry: The calculation of reactants and products in chemical reactions using balanced equations.

• Limiting Reactant: The reactant that is completely consumed first, limiting the amount of product formed.

• Theoretical Yield: The maximum amount of product that can be formed from the limiting reactant.

• Actual Yield: The amount of product actually obtained from a reaction.

• Percent Yield: The ratio of actual yield to theoretical yield, expressed as a percentage.

• Molarity (M): Moles of solute per liter of solution.

• ppm/ppb: Parts per million/billion, used for very dilute concentrations.

Additional info: Some examples and analogies (e.g., pizza recipe) are used to illustrate mole ratios and limiting reactants. All calculations should begin with conversion to moles for accuracy in stoichiometric analysis.