Why Percent Yield Compares Experiment with a Stoichiometric Maximum

Three yield quantities answer different questions

• Theoretical yield is the maximum product predicted by the balanced equation from the limiting reactant.
• Actual yield is the product obtained and measured after the experiment.
• Percent yield compares the measured result with the stoichiometric maximum.

$$\text{percent yield} =\frac{\text{actual yield}}{\text{theoretical yield}}\times100\%.$$

The numerator and denominator must describe the same product and use compatible units.

The limiting reactant sets the denominator

Theoretical yield is not chosen from whichever reactant has the larger mass. Convert each reactant to moles, apply the balanced-equation ratios, and identify which one can form less product. That reactant limits the theoretical yield.

Changing the limiting-reactant calculation changes the denominator and therefore changes percent yield, even if the measured product mass stays the same.

Example: compare a measured result with the maximum

Suppose stoichiometry predicts $42.0\,\mathrm g$ of a product, while isolation and drying produce $35.7\,\mathrm g$:

$$\text{percent yield} =\frac{35.7\,\mathrm g}{42.0\,\mathrm g}\times100\% =\mathbf{85.0\%}.$$

This percentage shows that the experiment recovered $85.0\%$ of the stoichiometric maximum. It does not identify the cause of the missing $15.0\%$.

Interpret unusual values carefully

Yields below $100\%$ can reflect incomplete reaction, side reactions, transfer losses, purification losses, or product remaining in solution. A reported yield above $100\%$ does not usually mean more product was created than stoichiometry permits; it is a cue to check for residual solvent, impurities, incomplete drying, measurement error, or an incorrect theoretical-yield calculation.

Percent yield is therefore both a performance measure and a diagnostic comparison. It does not by itself prove purity or reveal the mechanism of loss.