The percentage yield is a useful way of saying how much of a reactant has been successfully converted to product in a chemical reaction.
The actual yield is the amount of product you actually got after reacting the reactant together.
The theoretical yield is the amount of product you calculated by doing a balanced equation.
When you have a percentage equal to 100% this means that when you have written the balance equation, all the reactants are your product, but there are a lot of reasons this will not happen and get slower percentage. If you have a percentage yield like 25% this means a lot of your reactants reacted with something else which didn’t give the mass of your product you expected.
For example:
Alkenes can be prepared by the dehydration of alcohols with an acid catalyst. Cyclohexane can be prepared by the dehydration of cyclohexanol shown below.
A student reacted 7.65g of cyclohexanol, C6H12O, and obtained 0.0268 mol of cyclohexene.
Calculated the percentage yield of cyclohexane.
You can see that the equation is already balanced and given so you don’t need to worry about this but will sometimes need to do a balanced equation.
Your first step will be to find the moles of cyclohexanol with the mass already given and find the Mr.
C6H12O has a Mr of 100.
Moles=mass/Mr therefore you can do 7.65/100=0.0765 moles of cyclohexanol.
Your next step is to check the ratio of the equation between cyclohexene and cyclohexene. Here the ratio is 1:1 so cyclohexene has 0.0765 moles.
Now you can use the formula for percentage yield as we know the actual moles we got =0.0268mol and we know the theoretical moles we are supposed to get = 0.0765mol.
(0.0268/0.0765) x100=35.0%