Stoichiometry Step-by-Step and Formulas Summary

Step-by-Step Stoichiometric Calculation Process

  • Step 1: Balance the Chemical Equation

    • The first mandatory step in any stoichiometric calculation is balancing the reaction equation.
    • Balancing ensures that the number of atoms for each element is equal on both the reactant and product sides, satisfying the law of conservation of mass.

  • Step 2: Determine Molar Mass

    • Molar mass (MM) is calculated by taking the sum of the atomic masses of all individual elements contained within the chemical formula.
    • Molar mass is expressed in grams per mole (g mol−1g\,mol^{-1}).

  • Step 3: Calculate the Number of Moles

    • Convert the given mass of a substance into moles using the molar mass calculated in the previous step.
    • The fundamental formula for calculating moles (nn) from mass (mm) and molar mass (MM) is: Moles=MassMolar Mass\text{Moles} = \frac{\text{Mass}}{\text{Molar Mass}} \n n = \frac{m}{M}\n

  • Step 4: Identify the Limiting Reactant

    • To identify which reactant limits the extent of the reaction, divide the available molar amount of each reactant by its corresponding stoichiometric coefficient (the coefficient located directly in front of the chemical formula in the balanced equation): Ratio=Moles of ReactantStoichiometric Coefficient\text{Ratio} = \frac{\text{Moles of Reactant}}{\text{Stoichiometric Coefficient}}
    • Compare the calculated ratios for all reactants; the reactant that yields the smallest quotient is designated as the limiting reactant.

  • Step 5: Apply Proportional Calculations (Rule of Three)

    • Use exclusively the quantity of the identified limiting reactant to perform stoichiometric proportional calculations (rule of three).
    • Calculate the theoretical amount of product formed based solely on the limiting reactant, disregarding excess reactants.

Essential Stoichiometry Formulas

  • Percentage Yield Formula

    • The percentage yield (Yield %\text{Yield } \%) evaluates reaction efficiency by comparing the experimental yield obtained to the calculated theoretical yield: Yield %=(Obtained YieldTheoretical Yield)×100\text{Yield } \% = \left( \frac{\text{Obtained Yield}}{\text{Theoretical Yield}} \right) \times 100

  • Actual Mass Formula

    • The actual or real mass (Actual Mass\text{Actual Mass}) obtained from a reaction given a specific percentage yield and theoretical mass is calculated as: Actual Mass=Theoretical Mass×(Yield %100)\text{Actual Mass} = \text{Theoretical Mass} \times \left( \frac{\text{Yield } \%}{100} \right)

  • Sample Purity Formula

    • The mass of pure substance (Pure Mass\text{Pure Mass}) contained within an impure sample is calculated using the total mass of the sample and its percentage purity: Pure Mass=Total Mass×(Purity %100)\text{Pure Mass} = \text{Total Mass} \times \left( \frac{\text{Purity } \%}{100} \right)