First, we should understand what the percent yield (% yield) is and why it is important. The percent yield of a reaction is a measure of ho...
First, we should understand what the percent yield (% yield) is and why it is important. The percent yield of a reaction is a measure of how efficient the reaction is to produce a particular product.
The % yield of a reaction can be calculated by the equation below.
grams of product = grams of product after purification
theoretical yield = grams of the product assuming that 100% of the limiting reagent is consumed to generate the product.
The best way to explain the % yield is with an example. Let's consider a typical organic chemistry reaction, shown below.
Assume that we are using 2 moles of both starting materials (4-methylbenzoic acid and NBS), and after purification you got 322.50 g of 4-bromomethylbenzoic acid.
Since we are using the same amount of moles for both starting materials, you can think that they are consumed at the same time. Therefore, both can be considered as limiting reagents.
The theoretical yield is calculated by multiplying the amount of moles of the limiting reagent (in this case the amount of moles of any of the starting materials) and the molar mass of the product of interest.
The theoretical yield is 430.08 g. Now, we have all the information to calculate the % yield of the product, shown below.
Considerations
- To date chemical reactions are not 100% efficient. Therefore, we should expect the grams of the product to be smaller than the theoretical yield. If you get 100% yield or more that is suggesting the presence of impurities.
- Notice that the chemical reaction was not balanced, that is because in most organic reactions the coefficients are 1. Therefore, the ratio of reagents and products is 1:1.
- Catalysts are not considered when determining the limiting reagent.
