How to Calculate the Number of Moles From Two Limiting Reagents

Write out the equation describing your chemical reaction. To do so, you will write the reagents on the left-hand side, then a right-facing arrow, then the products. For example, the equation for the combustion of methane with oxygen to form carbon dioxide and water is CH4 + O2 --> CO2 + H2O.

Write numerical coefficients in front of some or all of the chemicals, as required, so the reaction equation is balanced. This means there are equal numbers of atoms of all elements on both sides. In the case of the example, you would add coefficients in front of oxygen and water as follows; CH4 + 2O2 --> CO2 + 2H2O. Each side of this equation now has equal numbers of C, H and O atoms.

Multiply the number of moles which you have of the first reagent by the ratio of the numerical coefficient of the other reagent to the first reagent's numerical coefficient. This tells you how many moles of the other reagent you require to use up all of the first reagent. For the example, if you had 2 moles of CH4, you would multiply 2 by 2/1 to obtain 4 moles. You require 4 moles of O2 to use up 2 moles of CH4.

Compare the value you just calculated to the number of moles of the other reagent you actually have. If the moles of the other reagent are less than your calculated value, that second reagent is the true limiting reagent. If it is more, the first reagent is limiting. In the case of the example, if you had only 3 moles of O2, it would be the limiting reagent.

Multiply the moles of your limiting reagent by the ratio of the numerical coefficient of the product you are interested in to the numerical coefficient of the limiting reagent. You have now calculated the moles of product which you will obtain from the reaction. For the methane example, if you wanted to know how many moles of CO2 would be produced, you would multiply 3 moles of O2 by 1/2 to obtain a value of 1.5 moles of CO2.