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# How to Calculate Theoretical Yields

❶Not Helpful 1 Helpful 1. Is the theoretical mass of water 15 grams or 2.   A theoretical yield is the amount of products created by a chemical reaction, provided none of the reactants were wasted and the reaction was fully completed.

Knowing the theoretical yield helps determine a reaction's efficiency. This is important to know at any level, from beginning chemistry students to industrial chemists seeking to maximize profits. The basic theoretical yield calculation starts with the chemical reaction equation, takes into account the molar amounts of reactants and products, and determines if enough of each reactant is present so they are all used up.

Determine the number of moles of each reactant. For solids, divide the mass of a reactant used by its molecular weight. For liquids and gases, multiply the volume by the density and then divide by the molecular weight. Multiply the molecular weight by the number of moles in the equation.

The reactant that has the smallest mole number is the limiting reagent. 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.

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. Each product has a theoretical yield, meaning the amount of product you would expect to get if the reaction is perfectly efficient.

The two products shown on the right are carbon dioxide and water. You can begin with either product to calculate theoretical yield. In some cases, you may be concerned only with one product or the other. If so, that is the one you would start with. Write down the number of moles of your limiting reactant. You must always compare moles of reactant to moles of product. 