How to calculate theoretical yield in grams?



Before starting a chemical reaction, it is better to evaluate the amount of product produced with the given amount of reactants. This process is called finding the theoretical yield. Besides finding the final concentrations of the resulting product, we can also use the same method to determine the quantity of reactant required to produce a specific product in the desired amount. 

Usually, the yield is calculated in moles. Since there’s not any balance that can measure the moles, we need to figure it in grams from the very first point. How is theoretical yield calculated, and how can we convert it into grams? Let’s find out!

You can also start to learn and solve theoretical yield by using a theoretical yield calculator chemistry.

Calculating theoretical yield for the given sample


Suppose you are asked to calculate the theoretical yield of a chemical reaction in which 10g of hydrogen gas is heated more than oxygen to form water. How would you predict the amount of water produced? Before going towards the calculation, let’s go through the chemical equation of this reaction.

H2 + O2 – – – > H2O

Step 1: 

In the very first step of finding the theoretical yield from the given sample, you need to make sure whether the chemical equation is balanced or not. As we can see from the equation mentioned above that it is not balanced since there are two hydrogen and two oxygen atoms on the left-hand side, while on the right-hand side, there are only one oxygen and two hydrogen atoms. Thus, it would help if you balanced the chemical equation first before proceeding further, such as

2H2 + O2 – – – 2H2O

Step 2:

After balancing the chemical equation, the next step is to find the mole ratio. Mole ratio is a stoichiometric proportion between the concentration of one component and the concentration of another element in the reaction. This value acts as a link between the reagents and products participating in a chemical reaction. Since in the given response, for every two hydrogen moles that have been used, two water moles are produced. Thus the mole ratio between these two components is one and will be written as 1 mol H2/1 mol H2O. And if you have the value in grams. You can also convert grams to moles by using the grams to moles calculator.

Step 3:

The information we have gathered yet is now enough to calculate the theoretical yield of a reaction in grams. All you need to do is to follow the given strategy

  1. Utilize the molar mass of reacting mixture and convert the number of reactants from moles to grams, such as

The molar mass of H2 = 2 grams

The molar mass of  H2O = 18 grams

  1. Derive the moles’ product by using the mole ratio between the component participating in the reaction medium, such as

Grams of H2O = grams H2 x (18 grams H2O/1 mol H2O)x (1 mol H2/2 grams H2) x (1 mol H2O/1 mol H2)

Suppose we have ten grams of hydrogen gas; after inserting the value into the equation, we get

Grams of H2O = 10grams H2 × (18 grams H2O/1 mol H2O) × (1 mol H2/2 grams H2) × (1 mol H2O/1 mol H2)

The Grams of H2O = (10g H2 × 18 × ½ × 1)

Grams of H2O = 90g H2O

Thus, we have found that when 10g of H2 reacts with oxygen access, it should produce 90g of water. Since it is a theoretical yield, the amount of product produced in real life might fluctuate a little. 

Calculating the number of reactants required to produce the desired product


We can use the same strategy to determine the quantities of reactants. However, a few modifications are made to the formula. Let’s reconsider the same reaction and change the question of how many grams of oxygen and hydrogen will be required to get 90g of water?

To know this, we will use the same formula of mole ratio but switch the reactants with the product as following

Grams of H2 = 90g H2O × (1 mol H2/1 mol H2O) × (1 mol H2O/18 g)×  (2 g H2/1 mol H2)

The Grams of H2 = (90g H2O × 1× 1/18 × 2)

Grams of H2 = 10g

The same method can be used to determine the amount of oxygen. Remember, finding the theoretical yield of a chemical reaction, either for reactants or products, is straightforward if you know how to balance a chemical equation to determine the mole ratio between reactants and products. 

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