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Calculating Reaction Yield and Percentage Yield from a Limiting Reactant

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  • 0:05 Theoretical and Actual Yield
  • 1:16 Percent Yield
  • 1:54 Example One
  • 3:50 Example Two
  • 5:22 Example Three
  • 6:28 Lesson Summary
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Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

Learn what the theoretical yield, actual yield and percent yield are. Given the limiting reactant, learn how to calculate the theoretical reaction yield, which is also known as the ideal reaction yield and percentage yield.

Theoretical and Actual Yield

When I make chocolate chip cookies, the recipe says that it will make 24 cookies. Unfortunately for those who like baked cookies, I tend to eat a lot of the dough, so what I actually end up with is closer to 20 cookies. The 24 cookies is the theoretical yield - this is what the recipe should make in a perfect world. The 20 cookies is my actual yield - the actual number of cookies I end up with.

In other lessons, you can learn how to make stoichiometry calculations, determine the limiting reactant and calculate the amount of product made in reactions. The calculations you learn there for the amount of product made give you the theoretical yield, also known as the ideal reaction yield. This means it is the maximum amount of product that can be made from the amount of reactants you started with. But, the real world isn't perfect. It isn't theoretical. You never actually get the theoretical yield. Instead, you get the actual yield. This is the amount you would actually measure at the end of a chemical reaction in the lab. Actual yield is the measured amount of a product from a reaction.

Percent Yield

The equation for calculating the percent yield of chemical reactions
Percent Yield Equation

In the lab, scientists vary the way they do reactions so they can maximize the yield they get. They have done the calculations and know the theoretical yield they should have gotten. They also know how much they actually got when they measured it. They compare the actual yield to the theoretical yield to get their percent yield. Their goal is to get a percent yield as close to 100% as they can, so this is an important calculation for them.

The equation for percent yield is percent yield = actual yield/theoretical yield x 100%.

Let me show you how this works with an actual chemical reaction.

Example One

If 100g of CO reacts with an excess of H2 to form 75g CH3 OH, what is the percent yield of the reaction?

The balanced equation is CO + 2 H2 = CH3 OH.

First, determine the theoretical yield. You have mass of the reactant CO, so you need to change this to moles of CO. Use the mole ratio and then determine the theoretical amount of CH3 OH that will be made.

Mass of CO (use the molar mass equivalent here) = moles CO (use the mole ratio here to change moles to moles) = moles CH3 OH (use the molar mass equivalent here) = mass CH3 OH= percent yield.

100g CO x 1 mole CO / 28g CO = 3.6 moles CO

3.6 moles CO x 1 mole CH3 OH / 1 mole CO = 3.6 moles CH3 OH

3.6 moles CH3 OH x 32g CH3 OH / 1 mole CH3 OH = 115.2 g CH3 OH. This is the theoretical yield.

Percent yield = actual yield/theoretical yield x 100%

Percent yield = 75g CH3 OH / 115.2 g CH3 OH x 100% = 65%

The percent yield for example one is determined by solving this equation.
Percent Yield Example One

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