Finding Theoretical Yield

What is Theoretical Yield?

The quantity of a product received from the complete conversion of the limiting reactant in a chemical process is known as theoretical yield. The amount of product produced by a flawless (theoretical) chemical reaction isn’t the same as the amount you’ll receive from a lab reaction. Theoretical yield is often measured in grammes or moles.

As opposed to theoretical yield, the actual yield is the amount of product produced by a reaction. An actual yield may be higher than a theoretical yield because a subsequent reaction provides more product or because the recovered product contains impurities.

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How to Find/Calculate Theoretical Yield?

It’s helpful to know how much product will be created with given quantities of reactants before initiating chemical reactions, known as the theoretical yield. This is an approach to employ when determining the theoretical yield of a chemical process. The same technique can determine how much of each reagent is needed to make a certain amount of product.

Calculation of the Theoretical Yield Sample

To make water, 10 grams of hydrogen gas are burned in the presence of excess oxygen gas. What is the total amount of water produced?

The reaction that produces water when hydrogen gas and oxygen gas combine is:

H2 (g) + O2 (g) → H2O (l)

Step 1: Check to see if your chemical equations are balanced.

The above equation is unbalanced. After balancing, the equation becomes:

2H2 (g) + O2 (g) → 2H2O (l)

Step 2: Calculate the mole ratios of the reactants and products.

The stoichiometric ratio between the quantity of one chemical and the amount of another compound is known as the mole ratio. This value serves as a link between the reactant and the finished product. H2 and H2O have a mole ratio of 1 mol H2/1 mol H2O.

Step 3: Find the theoretical yield of the reaction.

There is now sufficient information to calculate the theoretical yield. Use the following strategy:

  • To convert grams of reactant to moles of reactant, use the molar mass of the reactant
  • To convert moles reactant to moles product, use the mole ratio between reactant and product
  • To convert moles of product to grams of product, use the molar mass of the product.

Therefore, in equation form:

Grams product = grams reactant x (1 mol reactant/molar mass of reactant) x (mole ratio product/reactant) x (molar mass of product/1 mol product)

The reaction’s theoretical yield is computed as follows:

  • the molar mass of H2 gas = 2 grams
  • the molar mass of H2O = 18 grams

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

We have 10 grams of H2 gas, so:

∴ grams H2O = 10 g H2 x (1 mol H2/2 g H2) x (1 mol H2O/1 mol H2) x (18 g H2O/1 mol H2O)

Except for grams H2O, all units cancel out, leaving:

grams H2O = (10 x 1/2 x 1 x 18) grams H2O

grams H2O = 90 grams H2O

Thus, theoretically, ten grams of hydrogen gas and excess oxygen will make up 90 grams of water.

Example of Calculation

Calculate the theoretical yield of geranyl formate production from 375 g of geraniol. A chemist makes geranyl formate using 375g of raw material and 417g of refined product. The yield percentage is quoted as 94.1%.

Solution:

The actual yield is 417g, which is the target product quantity.

Percentage yield is 94.1%

∴ Theoretical yield = (Actual yield/percentage yield) x 100

= (417 / 94.1)100

= 443g

Limiting Reactant and Percent Yield

The limiting reactant of a balanced chemical equation is identified to determine theoretical yield. Because the limiting reactant isn’t found in abundance, the reaction can’t continue once used up.

To find the limiting reactant, the below points are to be remembered:

  • If the quantity of reactants is given in moles, convert the results to grams.
  • In grams per mole, divide the mass of the reactant by its molecular weight.
  • Alternatively, we can multiply the amount of a reactant solution in millilitres by its density in grams per millilitre for a liquid solution. Then divide the result by the molar mass of the reactant.
  • Multiply the mass obtained by the number of moles of the reactant in the balanced equation using either technique.
  • Now we know how many moles each reactant has. To determine which is accessible in excess and which will be used up first (the limiting reactant), compare this to the molar ratio of the reactants.

To calculate the percent yield, first establish the amount of product that should be produced using stoichiometry. This is the maximum amount of product made with reactant amounts available. When a reaction is carried out in the lab, the actual yield is the amount of produced product. The percent yield is the percentage difference between the actual and theoretical yields.

Percent Yield = Mass of Actual Yield / Mass of Theoretical Yield x 100 percent

In the manufacturing of items, the percent yield is essential. Chemical producers spend a lot of time and money improving their percent yield. When many separate reactions are used to make complicated molecules, one step with a low percent yield can quickly lose a lot of reactants and cost a lot of money.

Frequently Asked Questions on Finding Theoretical Yield

Q1

What method do you use to calculate the theoretical yield?

Multiply the ratio by the amount of the limiting reactant in moles. The theoretical yield of the desired product in moles is the answer.

Q2

What is the difference between theoretical and actual yield?

Theoretical yields are calculated amounts of products based on the complete reaction of the limiting reagent, whereas the actual yield is the amount of product produced.

Q3

What is the significance of the limiting reactant’s name?

A limiting reactant, or present in a set amount, is such because it prevents additional reactants from participating in the reaction, even if they are present in excess.

Q4

What is the effect of concentration on yield?

A change in pressure, temperature, or concentration, according to Le Châtelier’s Principle, will push the equilibrium to one side of the chemical equation. As a result, increasing the yield requires manipulating the environment to favour the product side.

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