In the real world, reactions do not occur under perfect conditions - this only occurs in the theoretical world.

Limiting Reagents

When conducting most reactions, the amount of one reactant added is rarely in perfect proportion to the amount of another reactant added.

By definition, the limiting reagent is the first reactant to be fully consumed, limiting the reaction from continuing to take place. Being able to identify the limiting reagent is important, because it will decide the number of moles of product(s) produced.

The most intuitive way of determining the limiting reactant is the comparison method. This involves determining the number of moles of each reactant present, then using the molar ratio to determine which one is in insufficient proportions.

For example, lets consider if we dissolved magnesium in a beaker of hydrochloric acid. In this scenario, we have 6g of solid Mg and 200mL of 1mol/L of HCl. We’ll use these quantities to determine the number of moles we have of each reactant…

With 0.247 mol of Mg and 0.200 mol of HCl, we can find the limiting reagent (the reactant that will be insufficient) by using the molar ratio. For this reaction, we have two options:

1. Calculate the number of moles of HCl required to react with 0.247 mol of Mg, or...

2. Calculate the number of moles of Mg required to react with 0.200 mol of HCl

   
   

Both calculations will tell us if the reactant we are focusing on is the limiting reagent or not.

As we need 0.494 mol of HCl and only have 0.200 mol, or as there is 0.247 mol of Mg and we only need 0.100 mol, we can deduce that HCl is the limiting reagent.

Then, if you wanted to determine the amount of each product you’ll end up with, you’d need to use the number of moles of the limiting reagent in your calculations.

Purity, Yield & Dilusion

In the real world, it is common to be working with reactants that are impure or conducting a reaction that is not completely efficient. You may also find aqueous reactants or products that are not of a desired concentration.

In these situations, you'd use the following equations:

Understanding how to use and rearrange each of these equations is essential for dealing with questions where purity, yield and dilusion are factors.