Why does percent yield not equal 100

The question asks for the yield in terms of a percent, specifically that above percent. A yield value in terms of percent is called percentage yield. Percentage yield is equal to the actual yield divided by the theoretical yield multiplied by percent.

Actual yield is the experimentally obtained or measured yield at the end of a chemical reaction. Theoretical yield is the maximum amount of product that can be obtained from a chemical reaction if all the reactants were converted to product. And this value is calculated. If a percentage yield obtained is above percent, this means that the value in the numerator of the percentage yield expression is larger than the value in the denominator. For an actual yield value to be higher than a theoretical yield value, this means that the experimentally obtained yield or the measured yield is larger than the calculated maximum possible yield.

But this does not seem possible. How can we get more product than the maximum possible amount? If actual yield is larger than theoretical yield, this can only mean one thing, a contaminated product.

In other words, the product contains something else as well adding to its mass. Percent yield is the percent ratio of actual yield to the theoretical yield. Reasons for this can include incomplete or competing reactions and loss of sample during recovery. This can happen when other reactions were occurring that also formed the product.

It can also be a source of error if the excess is due to incomplete removal of water or other impurities from the sample. Percent yield is always a positive value. Also Known As: percentage yield. The equation for percent yield is:. Units for both actual and theoretical yield need to be the same moles or grams. For example, the decomposition of magnesium carbonate forms 15 grams of magnesium oxide in an experiment.

The theoretical yield is known to be 19 grams. What is the percent yield of magnesium oxide? The calculation is simple if you know the actual and theoretical yields. All you need to do is plug the values into the formula:.

Usually, you have to calculate the theoretical yield based on the balanced equation. In this equation, the reactant and the product have a mole ratio , so if you know the amount of reactant, you know the theoretical yield is the same value in moles not grams! You take the number of grams of reactant you have, convert it to moles, and then use this number of moles to find out how many grams of product to expect. Much time and money is spent improving the percent yield for chemical production.

When complex chemicals are synthesized by many different reactions, one step with a low percent yield can quickly cause a large waste of reactants and unnecessary expense. When a chemist synthesizes a desired chemical, he or she is always careful to purify the products of the reaction. Potassium chlorate decomposes upon slight heating in the presence of a catalyst, according to the reaction below:.

First, we will calculate the theoretical yield based on the stoichiometry. Step 1: Identify the "given" information and what the problem is asking you to "find". Step 2: List other known quantities and plan the problem. Step 3: Apply stoichiometry to convert from the mass of a reactant to the mass of a product:. Step 5: Think about your result.

Now we will use the actual yield and the theoretical yield to calculate the percent yield. Step 3: Use the percent yield equation below. Step 4: Solve. Upon reaction of 1. Since the amount of product in grams is not required, only the molar mass of the reactants is needed. The provided information identifies copper sulfate as the limiting reactant, and so the theoretical yield g Cu is found by performing mass-mass calculation based on the initial amount of CuSO 4.

Using this theoretical yield and the provided value for actual yield, the percent yield is calculated to be:. What is the percent yield of a reaction that produces