L33. Percent Yield

Stoichiometry and Energy

R-report

L33. Percent Yield

How close did your experiment come to the maximum possible product?

Central Idea:

Percent yield compares the amount of product actually collected in an experiment with the theoretical amount predicted from a balanced chemical equation, and expresses that ratio as a percent. Theoretical yield is the ideal maximum product calculated by stoichiometry (using a balanced equation and molar masses); actual yield is the mass you isolate from the reaction. Use the formula percent yield = (actual yield ÷ theoretical yield) × 100 to calculate how efficient a reaction was. (Example: if stoichiometry predicts 10.0 g but you collect 8.0 g, the percent yield is 8.0 ÷ 10.0 × 100 = 80%.) Very common.

How To Observe Changes:

1. Measure and record masses carefully: weigh starting materials and final product (example: record reactant masses in grams). 2. Keep a separate list of observations (color change, solid formed) and explanations (reaction happened, side reaction); do not mix them. 3. Calculate theoretical yield from the balanced equation and molar masses before comparing to experiment. 4. Weigh the dry, filtered product only (remove solvent) and report units in grams. 5. Check your calculation by redoing molar conversions and unit steps; test for transfer loss (rinse and reweigh) or incomplete reaction (run reaction longer). Keep observations clear and dated for reproducibility always.

Apply To New Situations:

When you compare theoretical yield and actual yield, the chemical identity and stoichiometry stay the same while the measured mass and experimental efficiency change. Theoretical yield depends only on the balanced chemical equation, mole ratios, and molar masses (these do not change); actual yield depends on how the experiment was run, how completely reactants reacted, and how much product was recovered. Diagnostic question: Did you collect all of the solid product without loss during transfer or filtration (Yes/No)? To falsify an initial low-yield conclusion, test for missed product: examine the filtrate or washings for dissolved product, repeat the reaction longer to check for incomplete reaction, and measure masses before and after each step to find where mass was lost. Also test for side reactions by checking for unexpected colors or residues and by running a qualitative test for the expected product.

Synthesize Ideas:

Percent yield connects stoichiometry to lab results: theoretical yield comes from a balanced equation and molar masses, while actual yield is what you collect. Useful vocabulary includes theoretical yield, actual yield, percent yield, stoichiometry. Remember to show units in calculations (grams, moles) and to conserve atoms and charge in every chemical equation (for example, 2 H2 + O2 → 2 H2O is balanced). As illustrative calculation: percent yield = (8.0 g ÷ 10.0 g) × 100 = 80% (units cancel).

Check units and significant figures on every mass and mole calculation. Verify that the chemical equation is balanced so atoms and charge are conserved; correct any stoichiometry errors. Compare observations to explanations: ask whether color, mass change, or gas evolution match the proposed reaction. Look for experimental losses: transfers, incomplete reaction, or side products, and test the filtrate or residues. Repeat these reasoning steps on the next problem and record each checked value. Keep answers neat for future review.