6.7- Finding % yield 

 

 

Definition: 

Yield- the amount of the product that is obtained after a reaction. 

 

 

Up until now, all our reactions have taken place under ideal conditions.

 

However, ideal conditions don't exist in real life. Therefore, we would have to find two things: Theoretical yield and Actual yield.

 

 

 

 

Theoretical yield is the calculated (expected) amount from an experiment after the reaction. 

 

Actual yield is what you would actually get (hence the word, 'actual') after a reaction. 

 

The reason why we have these two amounts is because reactants may be impure, reactions may not go to completion, or given reactions may have to compete with several smaller side reactions. It could even be caused by human error, e.g leftover chemicals in test tubes from previous experiments.

 

In fact, in the laboratory, if you get 60% of the expected amount of product, that is considered very good.

 

Anyway, let's try out on solving these kind of questions.

 

First, let's start of with finding what is THEORETICALLY YIELD

 

Example question:

If 16.0g of CaCO3 are decomposed and 7.54g of CaO are obtained, what is the percent yield of CaO obtained?

 

CaCO3 --> CaO + CO2

 

To find this, we can use a simple formula:

 

%yield = actual mass obtained x 100% 

                   calculated mass

 

Step One:

Always check to make sure the equation is balanced. 

-in this case, the formula is already balanced

 

Step Two:

Calculate the theoretical yield of CaO

 

Remember!  

if you are trying to convert between compounds in a chemical reaction you must be in moles

 

So let's look at the problem

we need to convert 16.0g of CaCO3 to moles. We do this by using this diagram:

                                                    ÷Ar

                                Mass                            Mole          

                                                     xAr

 

So from here, we know that we need to find the relative molecular mass (RMM) in order to get the mole.

 Ca= 40

 C= 12  

 O= 16  

 

(40x1)+(12x1)+(16x3)= 100

 

Now we do the same to CaO

(40x1)+(16x1)= 56

 

Now to find number of moles in 16g, we must:

 

 

Mole  = mass

              Ar

 16

100 = 0.16 mol

 

0.16CaCO3 --> 0.16CaO + 0.16CO2

         |                    |

      16.0g              X

 

To find the mass of X, take the RMM and multiply it with 0.16.

56x0.16= 8.96g of CaO

 

 

Mass = mole x Ar

 

 

Therefore:

 

0.16CaCO3 --> 0.16CaO + 0.16CO2

         |                    |

      16.0g              8.96g

 

Now we have found the theoretical yield.

We now need to calculate the percentage of product obtained.

We can do this by using the formula from before:

 

 

%yield = actual mass obtained x 100% 

                   calculated mass

 

 

%yield =  7.54g  x 100% 

                8.96g

 

%yield = 84.2% (3 significant figures)

 

 

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