Reaction Yields

Identifying the Limiting Reagent

• Step 1: Convert the mass of all starting reactants into moles using their respective molar masses ($n = \frac{m}{M_r}$).
• Step 2: Divide the number of moles of each reactant by its coefficient from the balanced chemical equation.
• Step 3: The reactant with the smallest resulting value is the limiting reagent; all theoretical yield calculations must proceed using this substance.

Calculating Percentage Yield

• Step 1: Use the moles of the limiting reagent and the stoichiometric ratio to find the theoretical moles of the product.
• Step 2: Convert these theoretical moles into a theoretical mass ($m = n \times M_r$).
• Step 3: Apply the percentage yield formula:

Formula: $\text{Percentage Yield} = \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \times 100$

• Check the Units: Always ensure that the actual and theoretical yields are in the same units (both grams or both moles) before dividing, otherwise the percentage will be incorrect.

• The 100% Rule: If your calculated percentage yield is over 100%, you have made an error. This usually indicates the product is impure (e.g., still wet with solvent) or a calculation mistake was made in the molar masses.

• Identify the Limiting Reagent First: Never assume the reactant with the smallest mass is the limiting reagent. You must always convert to moles and account for the stoichiometric coefficients first.

• Significant Figures: Ensure your final percentage yield is rounded to the same number of significant figures as the least precise measurement provided in the question.

• Ignoring Coefficients: A common mistake is comparing the raw number of moles of reactants without dividing by their balancing coefficients. This leads to incorrectly identifying the limiting reagent.

• Reversible Reactions: Students often forget that in equilibrium systems, the reaction does not go to completion by design, which naturally results in a lower yield unless conditions are manipulated.

• Product Loss: Loss of product during 'transfer' (moving liquid from one beaker to another) is a major reason for low actual yields in laboratory settings.