Limiting reactants and calculating theoretical yield
Quantitative chemistry • Amount of substance and mass
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Definition: Limiting reactant and excess reactant
A limiting reactant is the substance that is completely consumed first during a chemical reaction. The limiting reactant restricts the amount of product that can form because stoichiometric ratios require fixed proportions of reactants. An excess reactant remains after the reaction finishes and does not determine the final amount of product. Identification of the limiting reactant requires comparison of the amounts of reactants relative to their stoichiometric coefficients.
Definition: Theoretical yield and actual yield
Theoretical yield is the maximum amount of product expected when the limiting reactant is fully consumed, calculated from the balanced equation. Actual yield is the measured amount of product obtained from an experiment. Percentage yield equals (actual yield / theoretical yield) × 100%. Theoretical yield uses mole ratios and ideal complete conversion; real experiments often give lower actual yields due to side reactions, incomplete reactions, or practical losses.
Effect of a limiting quantity in terms of moles
Balanced equations provide molar ratios that link moles of reactants to moles of products. Calculations start by converting given masses or amounts to moles. For each reactant, the available moles are divided by its stoichiometric coefficient to find the relative amount that can react. The reactant giving the smallest relative value is the limiting reactant. Theoretical moles of product follow directly from the molar ratio between limiting reactant and product. Cause → effect: smaller available mole ratio causes lower maximum moles of product.
Effect of a limiting quantity in terms of masses (grams)
Mass-based calculations convert masses to moles using Moles = mass / molar mass. The limiting reactant in mass terms is the substance whose converted mole amount yields the smallest proportional amount relative to its stoichiometric coefficient. After identifying the limiting reactant by moles, theoretical mass of product equals moles of product × product molar mass. Cause → effect: a smaller mass of limiting reactant (after molar conversion) causes a smaller theoretical mass of product.
Step-by-step calculation method
Step 1: Write and balance the chemical equation. Step 2: Convert known masses, volumes or concentrations into moles. Step 3: Use stoichiometric coefficients to calculate how many moles of product each reactant could produce. Step 4: Identify the limiting reactant as the one producing the fewest moles of product. Step 5: Calculate theoretical moles of product from the limiting reactant and convert to grams if required. Step 6: Use actual mass to find percentage yield if needed. Each step depends on correct balancing and accurate molar masses.
Common limiting factors and calculation pitfalls
Unbalanced equations lead to incorrect molar ratios and wrong limiting reactant selection. Using inconsistent units (grams vs moles) causes calculation errors. Rounding too early distorts answers; carry significant figures through the calculation and round at the end. Misidentifying the limiting reactant by comparing raw masses instead of mole-based ratios leads to incorrect theoretical yields. Cause → effect: improper equation balancing or unit errors causes false theoretical yields and incorrect percentage yields.
Key notes
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