Stoichiometry and Limiting Reactants

Stoichiometry: Relating Masses and Moles in Chemical Reactions

Stoichiometry is the quantitative relationship between reactants and products in a chemical reaction. It allows chemists to predict the amounts of substances consumed and produced using balanced chemical equations.

• Stoichiometric coefficients indicate the ratio in which substances react and are formed.

• Calculations often involve converting between mass and moles using molar mass, and relating moles of different substances using the balanced equation.

• Example: For the reaction , 1 mole of methane reacts with 2 moles of oxygen to produce 1 mole of carbon dioxide and 2 moles of water.

• Mole Road Map: The typical steps in stoichiometry are:

  1. Convert mass of substance A to moles (using molar mass).

  2. Use the balanced equation to convert moles of A to moles of B.

  3. Convert moles of B to mass (using molar mass).

Limiting and Excess Reactants

In reactions with more than one reactant, the limiting reactant is the substance that is completely consumed first, thus determining the maximum amount of product formed. The excess reactant is the substance left over after the reaction.

• To identify the limiting reactant:

  1. Calculate the amount of product formed from each reactant individually.

  2. The reactant that produces the least amount of product is the limiting reactant.

• To find the amount of excess reactant remaining:

  1. Calculate how much of the excess reactant is used by the limiting reactant.

  2. Subtract the used amount from the initial amount.

• Example: In the reaction , if 12.10 g F2 and 21.34 g KI are reacted, calculate the mass of KF produced and determine which reactant is limiting.

Percent Yield

Percent yield measures the efficiency of a reaction by comparing the actual (experimental) yield to the theoretical yield.

• Theoretical yield is the maximum amount of product that can be formed from the limiting reactant.

• Experimental yield is the actual amount of product obtained from the reaction.

• Percent yield formula:

• Percent yield can never exceed 100%. Values above 100% indicate measurement errors or impurities.

• Example: If 375 g Fe reacts with excess water to produce 200.0 g Fe3O4, and the theoretical yield is 518 g, then:

Multi-Step Reactions and Overall Yield

In reactions with multiple steps, the overall percent yield is the product of the yields of each step.

• Each step may have a different yield, and the overall yield is often significantly lower than the yield of individual steps.

• Example: If the first step has a 90% yield and the second step also has a 90% yield, the overall yield is or 81%.

Mass Percent Composition and Empirical Formula

Mass percent composition is the percentage by mass of each element in a compound or mixture.

• Mass percent formula:

• Empirical formula is the simplest whole-number ratio of elements in a compound, determined from mass percent data.

• Example: If a mixture contains 0.75 g Na in 2.00 g total, the mass percent of Na is .

Practice Problems and Applications

• Calculate the mass of product formed from given masses of reactants.

• Identify the limiting reactant and calculate the amount of excess reactant remaining.

• Determine percent yield from experimental and theoretical yields.

• Apply stoichiometry to multi-step reactions and mixtures.

Summary Table: Limiting Reactant and Percent Yield Calculations

Additional info:

• These notes expand on brief lecture points to provide full academic context, including definitions, formulas, and stepwise procedures for stoichiometry, limiting reactants, and percent yield.

• Practice problems and examples are included to reinforce concepts and applications.