Whenever we do experiments, the actual result is a little different from the result we predicted. In chemistry, this discrepancy is compared by calculating the percent yield. In this lesson, we will define percent yield and go over a few examples.
What Is Percent Yield?
If you’ve ever cooked a meal from a recipe, you will have noticed they often come with serving amounts, or a number that states how many people the recipe can feed. Sometimes, however, this number can be off, and you’ll wind up with either less or more food than you anticipated.
A number of things can account for this difference – food spillage prior to cooking, leaving ingredients too long on the stovetop, using the wrong measuring cup, and so forth.The same thing can happen whenever we perform experiments in the chemistry lab to make a compound. We first make calculations for how much of a compound we’ll end up with. However, these calculations are made under ideal conditions.
They do not account for experimental errors or personal errors of the experimenter. In the end, your prediction is often different from what you have actually made.In chemistry, we have theoretical yield, which is the amount of the product calculated from the limiting reactant. The limiting reactant is the reactant in the chemical reaction which limits the amount of product that can be formed. The actual yield is the actual amount produced when the experiment or reaction is carried out.The discrepancy between the theoretical yield and the actual yield can be calculated using the percent yield, which uses this formula:
|To use this formula for percent yield, you need to make sure that your actual yield and theoretical yield are in the same units. If the actual yield is in grams, then theoretical yield also needs to be in grams. If theoretical yield is in moles, then the actual yield also needs to be in moles. You will need to perform the conversions if you are given one measurement unit and you need the other.
Let’s look at an example. Magnesium carbonate (MgCO3) decomposed to form 15 grams of MgO in the actual experiment. If the theoretical yield is 19 grams, what is the percent yield of MgO?
Sometimes, you will also need to calculate the theoretical yield from the given chemical reaction.
To do this, you will need to convert the amount of your limiting reactant into the amount of your final product. Let’s take a look at how this is done with this problem.If the reaction of 30 grams of calcium carbonate (CaCO3) produces 15 grams of calcium oxide (CaO), what is the percent yield for the following reaction?
You look at your chemical reaction, and you see that your one and only limiting reactant is calcium carbonate. You can take your given amount of 30 grams of calcium carbonate and convert it into the amount of calcium oxide that would be formed under ideal conditions. That’s your theoretical yield. Take a moment and refresh your memory and your conversion skills if you need to before continuing.You end up with this calculation.