The limiting reagent is a chemical that sets how far a reaction will proceed before another chemical is consumed, which will cause the reaction to end. This is found by working out a balanced equation, comparing the number of units (mols) go into the reaction, and measuring how many mols of each will be used in the specified reaction. The chemical with less mols than the proportion requires will be the limiting reagent.

Let's look at this problem:

Calculate the theoretical yield of MgCl₂?

First, write out what you know about any component of the reaction:

To determine which is the limiting reagent, we need to compare them against one another. We need to compare them in moles so we'll need to convert each quantity to moles:

Randomly choose one of the reactants to determine how much of the other one you need to use up the one you selected. In this example, magnesium hydroxide is randomly chosen:

Now compare the mols of HCl that is needed to the mols of HCl that is available. In our example, we need 1.74 mol of HCl but only have 1.23 available. We do not have enough so HCl is the limiting reagent. Since HCl is the limiting reagent, use it to determine the theoretical yield of magnesium chloride (MgCl₂):

The theoretical yield of a reaction is the amount of product that would be formed if the reaction went to completion. Let's say we obtained 55.4 g of magnesium chloride in the lab. This is the actual yield and you would be provided this information in the problem. Now, let's determine the percent yield using the formula below: