Question

How can I find the percentage yield?

Answer 1

Percent yield represents the ratio between what is experimentally obtained and what is theoretically calculated, multiplied by 100%.

`"% yield" = ("actual yield")/("theoretical yield") * 100%`

So, let's say you want to do an experiment in the lab. You want to measure how much water is produced when 12.0 g of glucose (`C_6H_12O_6`) is burned with enough oxygen.

`C_6H_12O_6 + 6O_2 -> 6CO_2 + 6H_2O`

Since you have a `1:6` mole ratio between glucose and water, you can determine how much water you would get by

`12.0` `"g glucose" * ("1 mole glucose")/("180.0 g") * ("6 moles of water")/("1 mole glucose") * ("18.0 g")/("1 mole water") = 7.20g`

This represents your theoretical yield. If the percent yield is 100%, the actual yield will be equal to the theoretical yield. However, after you do the experiment you discover that only 6.50 g of water were produced.

Since less than what was calculated was actually produced, it means that the reaction's percent yield must be smaller than 100%. This is confirmed by

`"% yield" = ("6.50 g")/("7.20 g") * 100% = 90.3%`

You can backtrack from here and find out how much glucose reacted

`"65.0 g of water" * ("1 mole")/("18.0 g") * ("1 mole glucose")/("6 moles water") * ("180.0 g")/("1 mole glucose") = 10.8g`

So not all the glucose reacted, which means that oxygen was not sufficient for the reaction - it acted as a limiting reagent.

As a conclusion, percent yield problems always have one reactant act as a limiting reagent, thus causing a difference between what is calculated and what is actually obtained. A percent yield that exceeds 100% is never possible, under any circumstances, and means that errors were made in the calculations.