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How to Figure the Theoretical Yield of a Chemical Reaction

Chemists use reaction equations to show which chemicals react together and also the resulting products. These equations also give the relative ratios of atoms or molecules which combine and which are formed. If you know the amount of starting reactants, you can use the ratios in a reaction equation to calculate the amount of product you should get if everything works perfectly. This amount is known as the theoretical yield. In practice, it is very rare to get the theoretical yield from a reaction, but this is still a useful value to know since it can be applied to find out how efficient the reaction is.

Things You'll Need

Calculator
Reactant masses
Reactant/product molecular weights

Instructions

- 1
  Write out the equation for the chemical reaction. You can obtain this from a reference source such as a school lab manual, or a chemistry text book. For example, the equation for the combustion of methane gas is; CH4 + O2 --> CO2 + H2O and you might want to find the theoretical yield of carbon dioxide (CO2) if you react 16g CH4 and 54g O2.
- 2
  Add numerical coefficients in front of the chemical reactants or products as required to balance the equation. This means that there are as many atoms of each element on the left side as there are on the right side. The example equation is not balanced since there are 4 H and 2 O on the left and 2 H and 3 O on the right. You can rewrite the equation as CH4 + 2O2 --> CO2 + 2H2O to balance it.
- 3
  Calculate the moles of each reactant chemical by dividing the mass of each reactant by its molecular weight (MW) in grams per mole. A mole is a standard unit of chemical quantity equal to 6.02 x 10^23 atoms or molecules. You can find molecular weights in a basic reference source such as a textbook. If you had 16 g of CH4 (MW 16 g/mole) and 54 g O2 (32 g/mole), you would have 16/16 or 1 mole of CH4 and 54/32 or 1.69 moles O2.
- 4
  Find the limiting reactant in the reaction. This is the reacting chemical which will run out first as the reaction progresses and you can find it by comparing the moles of reactants based on their numerical coefficients. In the case of the example, you have 1 mole of CH4. Since 1 mole of CH4 reacts with 2 moles O2, you need 2 moles O2 to use this up, but you only have 1.69 moles, so O2 is the limiting reactant and will run out first.
- 5
  Multiply the moles of your limiting reactant by the ratio of the numerical coefficient of the product you are calculating yield for to the numerical coefficient of the limiting reactant. This gives you the moles of product which will theoretically be formed. For the example, you would multiply 1.69 moles O2 by the ratio of 1/2 to obtain a yield of 0.845 moles CO2.
- 6
  Multiply the result of your last calculation by the molecular weight of the product chemical to obtain the theoretical yield of the reaction in units of grams product. In the case of the example, you would calculate 0.845 g CO2 times 44 g/mole to obtain a theoretical yield of 37.2 g CO2.

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