How to Calculate Theoretical Yields

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A theoretical yield is the amount of products created by a chemical reaction, provided none of the reactants were wasted and the reaction was fully completed. Knowing the theoretical yield helps determine a reaction's efficiency. This is important to know at any level, from beginning chemistry students to industrial chemists seeking to maximize profits. The basic theoretical yield calculation starts with the chemical reaction equation, takes into account the molar amounts of reactants and products, and determines if enough of each reactant is present so they are all used up.

Step 1

Determine the number of moles of each reactant. For solids, divide the mass of a reactant used by its molecular weight. For liquids and gases, multiply the volume by the density and then divide by the molecular weight.

Step 2

Multiply the molecular weight by the number of moles in the equation. The reactant that has the smallest mole number is the limiting reagent.

Step 3

Calculate the theoretical mole yield by using the chemical equation. The multiply the ratio between the limiting reagent and the product by the number of moles of the limiting reagent used in the experiment. For example, if your equation was 4Al + 3O2 yields 2 Al2O3, and Al was your limiting reagent, you would divide the number of Al moles used by two because it takes four moles of Al to make two moles of Al2O3, a ratio of two to one.

Step 4

Multiply the number of moles of the product by the molecular weight of the product to determine the theoretical yield. For example, if you created 0.5 moles of Al2O3, the molecular weight of Al2CO3 is 101.96 g/mol, so you would get 50.98 grams as the theoretical yield.

Tips

  • Make sure you use units consistently; don't mix English and standard units.

References

About the Author

Mark Kennan is a writer based in the Kansas City area, specializing in personal finance and business topics. He has been writing since 2009 and has been published by "Quicken," "TurboTax," and "The Motley Fool."

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