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How to Calculate Theoretical Yields

❶Compare the ratios to find the limiting reactant. This is the theoretical yield of the equation.




In a true experiment, you are likely to lose some through the inefficiency of the experiment itself. To calculate theoretical yield, start by finding the limiting reactant in the equation, which is the reactant that gets used up first when the chemical reaction takes place.

Then, write down the number of moles in the limiting reactant. Next, divide the number of molecules of your desired product by the number of molecules of your limiting reactant to find the ratio of molecules between them. Then, multiply the ratio by the limiting reactant's quantity in moles. Finally, convert your answer to grams.

Start with a balanced chemical equation. A chemical equation is like a recipe. It shows the reactants on the left side reacting to form products on the right side. A properly balanced equation will show the same number of atoms going into the equation as reactants as you have coming out in the form of products. There are two atoms of hydrogen on both the left and right. But there are two atoms of oxygen going in as a reactant and only one atom in the product on the right.

This change has corrected the oxygen, which now has two atoms on both sides. But you now have two atoms of hydrogen on the left with four atoms of hydrogen on the right. Double the hydrogen in the reactant. This change now has 4 atoms of hydrogen on both sides, and two atoms of oxygen. The equation is balanced. As a more complicated example, oxygen and glucose can react to form carbon dioxide and water: Read this guide if you wish to review balancing chemical equations more thoroughly.

Calculate the molar mass of each reactant. Using the periodic table or some other reference, look up the molar mass of each atom in each compound. Add them together to find the molar mass of each compound of reactant. Do this for a single molecule of the compound.

Consider again the equation of converting oxygen and glucose into carbon dioxide and water: If necessary, you can find more precise values. To review this step in more detail, you can review Calculate Molar Mass.

Convert the amount of each reactant from grams to moles. For an actual experiment, you will know the mass in grams of each reactant that you are using. Divide this value by that compound's molar mass to convert the amount to moles.

Determine the molar ratio of the reactants. A mole is a tool used in chemistry to count molecules, based on their mass. By determining the number of moles of both oxygen and glucose, you know how many molecules of each you are starting with.

To find the ratio between the two, divide the number of moles of one reactant by the number of moles of the other. Thus, the ratio of oxygen to glucose molecules is 1. This ratio means that you have 9 times as many molecules of oxygen as you have of glucose. Find the ideal ratio for the reaction. Look at the balanced equation for the reaction. The coefficients in front of each molecule tell you the ratio of the molecules that you need for the reaction to occur.

If you use exactly the ratio given by the formula, then both reactants should be used equally. The coefficients indicate that you need 6 oxygen molecules for every 1 glucose molecule.

Compare the ratios to find the limiting reactant. In most chemical reactions, one of the reactants will be used up before the others. The one that gets used up first is called the limiting reactant. This limiting reactant determines how long the chemical reaction can take place and the theoretical yield you can expect. Compare the two ratios you calculated to identify the limiting reactant: The formula tells you that your ideal ratio is 6 times as much oxygen as glucose.

Therefore, you have more oxygen than required. Thus, the other reactant, glucose in this case, is the limiting reactant. Review the reaction to find the desired product. The right side of a chemical equation shows the products created by the reaction. The coefficients of each product, if the reaction is balanced, tells you the amount to expect, in molecular ratios. Some problems will give you this equation, while others ask you to write it out yourself, such as for a word problem.

Since atoms are not created or destroyed during a chemical reaction, each element should have the same number of atoms on the left and right side. The equation is balanced. Read this guide if you are asked to balance an equation yourself. Calculate the molar mass of each reactant. Look up the molar mass of each atom in the compound, then add them together to find the molar mass of that compound.

Do this for a single molecule of the compound. You can find a more precise value on a periodic table. Convert the amount of each reactant from grams to moles. Now it's time to look at the specific experiment you are studying. Write down the amounts of each reactant in grams. Divide this value by that compound's molar mass to convert the amount to moles. Find the ratio of your reactions.

A mole is an exact number for the amount of a substance and it is equal to 6. You now know how many molecules of each reactant you started with. Divide the moles of 1 reactant with the moles of the other to find the ratio of the 2 molecules.

You started with 1. The ratio of oxygen to glucose molecules is 1. This means you started with 9 molecules of oxygen for every 1 molecule of glucose. Find the ideal ratio for the reaction.

Go back to the balanced equation you wrote down earlier. This balanced equation tells you the ideal ratio of molecules: The coefficients tell you there are 6 oxygen molecules and 1 glucose molecule. Make sure you list the reactants in the same order you did for the other ratio. In a chemical reaction, 1 of the reactants gets used up before the others. The quantity of the product that is created in the reaction is limited by the reagent. Compare the 2 ratios you calculated to identify the limiting reactant: The bottom reactant in the ratio is the limiting reactant.

If the actual ratio is smaller than the ideal ratio, you don't have enough of the top reactant, so it is the limiting reactant. The bottom reactant, glucose, must be the limiting reactant. Identify your desired product. The right side of a chemical equation lists the products created by the reaction. Each product has a theoretical yield, meaning the amount of product you would expect to get if the reaction is perfectly efficient.

The right-hand side lists 2 products, carbon dioxide and water. Write down the number of moles of your limiting reactant. The theoretical yield of an experiment is the amount of product created in perfect conditions. To calculate this value, begin with the amount of limiting reactant in moles. This process is described above in the instructions for finding the limiting reactant. In the example above, you discovered that glucose was the limiting reactant.

You also calculated that you started with 0. Find the ratio of molecules in your product and reactant. Return to the balanced equation. Divide the number of molecules of your desired product by the number of molecules of your limiting reactant. In other words, this reaction can produce 6 molecules of carbon dioxide from 1 molecule of glucose. Multiply the ratio by the reactant's quantity in moles. The answer is the theoretical yield of the desired product in moles.

You started with 0. The theoretical yield of carbon dioxide is 0. Convert the result to grams. Multiply your answer in moles by the molar mass of that compound to find the theoretical yield in grams. This is a more convenient unit to use in most experiments. The theoretical yield of the experiment is The theoretical yield you calculated assumes that everything went perfectly.

In an actual experiment, this never happens: This is why chemists use 3 different concepts to refer to yield: The theoretical yield is the maximum amount of product the experiment could make. The actual yield is the actual amount you created, measured directly on a scale. Write down the actual yield of the experiment. If you performed the experiment yourself, gather the purified product from your reaction and weigh it on a balance to calculate its mass.

If you are working on a homework problem or someone else's notes, the actual yield should be listed. Divide the actual yield by the theoretical yield. Make sure you use the same units for both values typically grams. Your answer will be a unit-less ratio. The actual yield was 29 grams, while the theoretical yield was Multiply by to convert to a percentage.


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Before performing chemical reactions, it is helpful to know how much product will be produced with given quantities of reactants. This is known as the theoretical yield. This is a strategy to use when calculating theoretical yield of a chemical reaction.

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To determine theoretical yield, multiply the amount of moles of the limiting reagent by the ratio of the limiting reagent and the synthesized product and by the molecular weight of the product.

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Calculate the theoretical mole yield by using the chemical equation. The multiply the ratio between the limiting reagent and the product by the number of moles of the limiting reagent used in the experiment. You determine percent yield with the following formula: Lovely, but what is an actual yield, and what is a theoretical yield? An actual yield is, well, the amount of product actually produced by the reaction in a lab or as told to you in the chemistry problem.

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(reactants) available. This smallest yield of product is called the theoretical yield. To find the limiting reagent and theoretical yield, carry out the following procedure: 1. Find the moles of each reactant present. 2. Calculate the moles of a product formed from each mole of reactant. 3.