What is percentage yield and its types, how can we calculate it?

Yield:

In Chemistry or Chemical Engineering, we often study the processes in which a certain amount of chemical reactants, under specified reaction conditions are converted into products. 

The products are categorized as desired and underside products while the reactants are written as limiting reaction and excessive reactant. 

Reaction yield is the number of products that are obtained after successful completion of the chemical reactions. 

Yield is often calculated as the weight in grams and sometimes in moles depending upon the nature of reactions that may be solid, liquid or gas that take part in the reaction. If we study Vogel’s textbook of Practical Organic Chemistry, then there are various categories of yield that are explained in it.

It is mentioned that yields less than 40% are poor yields, above 70% are good and close to 100% are written as quantitative yields. 

Percentage Yield:

The ratio between actual yield and theoretical yield is called percentage yield. It is a dimensionless quantity and is often written in percentage (%). 

In another definition, percentage yield is calculated by dividing experimental yield by theoretical yield multiplied by 100. It is worth mentioning here that the percentage yield of chemical reaction usually doesn’t come out to be 100%.

The reason behind this fact is that actual yield is always less than theoretical yield due to either spilling of reactants or incomplete execution of the reaction. Hence, relatively lesser number of products are formed that too depends upon the limiting reactant. 

Moreover, percentage yields greater than 100% are only possible when there are certain impurities that get mixed with the final product. Hence, in this case the overall weight of the measured product is higher and it causes the amount of percentage yield to be increased beyond cent percentage. 

Percentage Yield Formula:

The percentage yield is calculated by the formula as under. 

Percentage Yield=Actual YieldTheoretical Yield×100

Where:

  • Actual Yield is the total amount of products obtained as a result of chemical reaction.
  • Theoretical Yield is the pre calculated amount of products via stoichiometric calculations and studying about limiting reactants. 
  • It should be noted here that both actual yield and theoretical yields should be in the same units either grams of moles otherwise percentage yield can not be calculated accurately. 

Percentage yield is a very important factor that decides the efficiency of the process that is being used in various chemical industries. 

The chemist and chemical engineers try their best to improve the percentage yield of their products so that maximum profit can be made after sales. 



Calculation of Percentage Yield:

Let us understand the concept of how to calculate percentage yield by studying an example. Potassium chlorate (KClO3) is decomposed into potassium chloride (KCl) and oxygen (O2) in the presence of heat and catalyst. 

It is a single step reaction in which one reactant is decomposed into two products. In a certain reaction 40g of KClO3 is decomposed into KCl and O2. The mass O2 calculated as products is around 15g. 

What is the percentage yield of reaction?  

The said problem can be solved in some steps stated below:

Step 1: Balanced Chemical Equation

2KClO3↔2KCl + 3O2

Step 2: Theoretical Yield of O2

Mass of KClO3 40 g

Molar Mass of KClO3 122.5 g/mole

Molar Mass of O2: 32g/mole

Molar Mass of KCl: 74 g/mole

Theoretical Yield of O2= 40 g KClO3 ×1 mole of KClO3 122.5 g KClO3 ×3 mol of O22 mol of KClO3 ×32 g O21 mol O2

Theoretical Yield of O2= 15.7 g O2

Hence, by taking into account the mass of the reactant and the molar masses of O2 the theoretical yield is calculated to be 15.7 g. 

Step 3: Percentage Yield: 

Now from the data given in the statement and we calculated above, we have

Actual Yield of O2= 15 g.

Theoretical Yield of O2= 15.7 g

Percentage Yield=Actual YieldTheoretical Yield×100

Percentage Yield=1515.7×100

Percentage Yield=95%

It is worth mentioning here that, we calculated the percentage yield of the reaction over O2 as the product, but we have another product in the chemical reaction. 

A very similar procedure can be adopted for calculating the percentage yield over KCl. The overall steps remain the same i.e. balanced chemical equation, calculation of theoretical yield and finally calculation of percentage yield.

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