Theoretical Yield for Organic Chemistry Labs

You weigh out 0.85 g of salicylic acid, add 2.0 mL of acetic anhydride, and heat the mixture. Your lab report asks for the theoretical yield. The answer begins with the limiting reactant, not the one you measured first. Organic chemistry lab reports demand theoretical yield calculations that work with millimoles, liquid densities, and reagent equivalents. Here is the process for a typical lab report, with the numbers you need.

Organic lab procedures almost never list masses in grams alone. They give amounts in millimoles (mmol) and use equivalents to describe how much of each reagent is present relative to the limiting reactant. One mmol is 0.001 mol. Converting to mmol keeps the numbers manageable and avoids unit mismatches. For a reagent listed as 1.5 equivalents, you multiply the mmol of the limiting reactant by 1.5 to get the mmol needed. The procedure tells you the equivalents. Your job is to confirm which reagent is limiting before you calculate theoretical yield.

Convert every reagent to mmol before you compare them. Weigh a solid on an analytical balance to 0.001 g and divide by the molar mass in g/mol, then multiply by 1000 to get mmol. For a liquid, you reach for the density.

Liquids are measured by volume, so you need density to get mass. The formula is mass (g) = volume (mL) × density (g/mL). Divide that mass by the molar mass to get moles, then multiply by 1000 for mmol. Use densities from the CRC Handbook of Chemistry and Physics or PubChem. For ethanol at 20 °C, density is 0.789 g/mL. For acetic acid, density is 1.049 g/mL. If the lab temperature is 22 °C instead of 20 °C, the density shifts by roughly 0.1-0.5%, which is negligible for a student calculation but worth noting in a research notebook.

Always check the purity line on the reagent bottle. A 95% pure liquid means you multiply the mass by 0.95 before converting to moles. This step is the most common source of error in liquid measurements.

Solvents, catalysts, and reagents labelled as 'excess' never become the limiting reactant. A solvent like dichloromethane (density 1.325 g/mL) used to dissolve the reactants is present in many times the stoichiometric amount. A catalyst such as concentrated sulfuric acid in an esterification is used in catalytic quantity and consumed only in trace amounts. Ignore these when calculating theoretical yield. Only the stoichiometric reagents that appear in the balanced equation with a defined mole ratio matter. The excess reactant remains after the reaction stops and does not factor into the maximum product.

If the procedure says 'use 2.0 equivalents of triethylamine,' the triethylamine is in excess unless the limiting reactant runs out first. Confirm by converting to mmol. The limiting reactant gives the smallest number of mmol divided by its own coefficient in the balanced equation. This is the limiting reactant definition used in OpenStax Chemistry 2e.

Start every theoretical yield calculation by building a reagent table. List the molecular weight (MW) from CIAAW standard atomic weights, the amount used (mass or volume), the mmol calculated, and the equivalents relative to the limiting reactant. Fill it in as you go. Use the table below for any organic procedure.

A Fischer esterification uses acetic acid (MW 60.05 g/mol) and ethanol (MW 46.07 g/mol) with a catalytic amount of sulfuric acid to produce ethyl acetate (MW 88.11 g/mol) and water. The balanced equation is CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O. You measure 2.5 mL of acetic acid (density 1.049 g/mL, 100% purity) and 3.0 mL of ethanol (density 0.789 g/mL, 95% purity). Convert acetic acid: 2.5 mL × 1.049 g/mL = 2.62 g. 2.62 g / 60.05 g/mol = 0.04363 mol = 43.63 mmol. Convert ethanol: 3.0 mL × 0.789 g/mL = 2.37 g. Multiply by 0.95 purity: 2.37 × 0.95 = 2.25 g. 2.25 g / 46.07 g/mol = 0.04884 mol = 48.84 mmol. The reaction is 1:1. Acetic acid gives 43.63 mmol of product; ethanol gives 48.84 mmol. Acetic acid is limiting. Theoretical yield of ethyl acetate = 43.63 mmol × 88.11 g/mol / 1000 = 3.84 g. A typical student achieves 60-80% after distillation. The percent yield formula, (actual yield / theoretical yield) × 100%, converts your recovered mass into a percentage.

In a common undergraduate aspirin synthesis, salicylic acid (MW 138.12 g/mol) reacts with acetic anhydride (MW 102.09 g/mol, density 1.082 g/mL) to produce aspirin (MW 180.16 g/mol) and acetic acid. The balanced equation is C₇H₆O₃ + (CH₃CO)₂O → C₉H₈O₄ + CH₃COOH. You weigh 2.00 g of salicylic acid. You add 4.0 mL of acetic anhydride. Convert salicylic acid: 2.00 g / 138.12 g/mol = 0.01448 mol = 14.48 mmol. Convert acetic anhydride: 4.0 mL × 1.082 g/mL = 4.33 g. 4.33 g / 102.09 g/mol = 0.04241 mol = 42.41 mmol. The reaction is 1:1. Salicylic acid is limiting. Theoretical yield of aspirin = 14.48 mmol × 180.16 g/mol / 1000 = 2.61 g. If your recovered mass after recrystallization is 2.05 g, the percent yield is (2.05 / 2.61) × 100% = 78.5%. This is a good yield for a teaching lab. A detailed procedure would also account for the 1:1 mole ratio and the need to dry the product completely before weighing.

In a research synthesis, more than one reaction step is typical. The overall theoretical yield for a multistep synthesis is the product of each step's yield, expressed as a decimal. If step one has a theoretical yield of 85% (0.85), step two yields 90% (0.90), and step three yields 72% (0.72), the overall yield is 0.85 × 0.90 × 0.72 = 0.5508, or 55.1%. This assumes no losses between steps. In practice, each extraction, filtration, and column chromatography step costs product. A three-step synthesis with 90% per step gives 72.9% overall. A five-step synthesis with the same per-step yield gives only 59.0%. This is why synthetic chemists aim for 80-90% per step to keep the overall yield above 50% after four or five steps.

Calculate the theoretical yield for each step independently using the limiting reactant of that step. Multiply the step theoretical yields to get the overall theoretical yield. Note that the percent yield of an individual step is reported relative to that step's own theoretical yield, not the original starting mass. This is the standard reporting method in organic chemistry lab notebooks and journal publications.

Every lab notebook entry that involves a synthesis should include a theoretical yield calculation. Write the balanced equation. List each reagent with its mass or volume, molar mass, mmol, and equivalents in a table. Identify the limiting reactant. Show the mmol-to-grams conversion for the product. After the experiment, record the actual yield in grams and calculate the percent yield. A percent yield above 100% means the product is not dry or contains impurities. A percent yield below 10% means the reaction did not work or the product was lost during workup. The yield is never the same as the theoretical yield.

Include the source of your density and molar mass data. Write 'density from CRC Handbook, 20 °C' or 'MW from CIAAW 2021 values.' Organic lab instructors expect this attribution. Do not round intermediate values until the final step. Carry four significant figures through the calculations and round the final theoretical yield and percent yield to three significant figures.