Calculate percentage yield from actual and theoretical yield
Quantitative chemistry • Yield and atom economy
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Key concepts
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Theoretical yield
Theoretical yield equals the maximum amount of product that stoichiometry predicts when the limiting reagent reacts completely. Calculation requires balanced chemical equations, conversion of given masses to moles, and identification of the limiting reagent. Theoretical yield can be expressed in moles or mass; conversion from moles to mass uses relative formula mass (Mr) or molar mass. Accurate theoretical-yield calculation prevents overestimation of reaction efficiency. Errors in stoichiometry, incorrect molar masses or failure to identify the limiting reagent cause incorrect theoretical yields and therefore incorrect percentage yields.
Actual yield
Actual yield equals the mass or number of moles of product collected in an experiment. Measurement uses balances for mass or calculations from concentrations and volumes for solutions. Actual yield must use the same units as theoretical yield when calculating percentage yield; conversion between mass and moles is necessary when units differ. Experimental conditions and procedural steps determine actual yield. Losses during filtration, transfer, evaporation or product purification directly reduce actual yield and lower percentage yield.
Percentage yield formula
Percentage yield equals (actual yield ÷ theoretical yield) × 100%. The formula produces a dimensionless percentage that indicates reaction efficiency. Both actual and theoretical yields must refer to the same substance and use identical units before applying the formula. A percentage yield greater than 100% indicates measurement error, impure product or incorrect theoretical-yield calculation. A percentage yield of 100% indicates complete conversion of limiting reagent with no losses.
Limiting reagent
The limiting reagent is the reactant that runs out first and therefore limits the amount of product formed. Determination requires comparing mole ratios of reactants to mole ratios in the balanced equation. The reactant producing the smaller amount of product (in moles) is the limiting reagent. Identification of the limiting reagent directly affects the theoretical yield. Incorrect identification leads to incorrect theoretical yield and invalid percentage-yield results.
Common causes of low percentage yield
Incomplete reactions cause unreacted reactants to remain, which reduces the actual amount of product. Side reactions produce by-products that consume reactants and lower the yield of the desired product. Practical losses such as spillage, filtration loss or transfer losses remove product from collection and lower actual yield. Impure reactants and impure product produce incorrect mass readings. Measurement errors on balances or inaccurate concentration measurements for solutions also affect actual yield and therefore affect percentage yield.
Units, rounding and significant figures
Percentage yield calculation requires consistency of units; convert moles to mass or mass to moles before applying the formula. Rounding should preserve at least two significant figures more than the final required precision to avoid large relative errors. Report percentage yield to an appropriate number of significant figures based on the precision of measured data. Exceeding correct precision or failing to convert units produces misleading percentage-yield values. Careful unit handling and consistent rounding maintain reliability of reported yields.
Key notes
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