Question

A reaction is said to have a percentage yield of 80%. If the actual yield is 25gm what was the expected yield?
1. $3.2grams$
2. $19grams$
3. $31.25grams$
4. $190grams$

Answer 1

Percentage yield can be defined as the ratio of actual yield to the expected yield multiplied by $100$.
$Percentage\,Yield = \dfrac{{Actual\,yield}}{{Theoretical\,yield}} \times 100$
Theoretical yield is the same as expected yield. Use the above formula and substitute appropriate values given in the question to find the expected yield.

Complete step by step solution:
Let us first understand the terms which are being used in the above questions.
Theoretical yield is the amount of product we are supposed to get when carrying out a reaction. We do these calculations by simple Stoichiometric calculation or sometimes by complex methods. In every reaction we can determine the yield of the product by knowing the amount of reactant we have added.
Actual Yield is the practical weight of the product we take after carrying out the reaction in the lab. This weight can be taken in a weighing balance. In Ideal conditions the Actual Yield and Theoretical Yield should be the same, but that is not the case. There are errors which happen when we proceed with the reaction, these errors could be because of the scientist, that they made an error while performing the experiment or it could be due to the molecules such as all the molecules of the reactant did not take part in the reaction. In any case, the yield we get will be less than the yield we expected.
The formula for percentage yield is: $Percentage\,Yield = \dfrac{{Actual\,yield}}{{Theoretical\,yield}} \times 100$
From the above question:

$Percentage\,Yield = \,80\%$ $Actual\,Yield = \,25grams$
Substituting these values in the formula we get:
$80 = \dfrac{{25}}{{Theoretical\,yield}} \times 100$
$\therefore Theoretical\,Yield = 31.259grams$
Hence, Option 3 is the correct answer.

Note:
Actual Yield will always be less than the theoretical yield. This is because ideal reaction conditions are not possible to maintain. If the Actual Yield is greater than the theoretical yield, the percentage yield will be $> 100\%$ which is not possible mathematically or practically.