A Guide to Conquering Stoichiometry

Stoichiometry can be your first enemy in Chemistry, but it can be your greatest friend with practice. This topic in chemistry is fundamental and will come to you comfortably as you progress in the subject.   

Let’s first understand what stoichiometry is.

Consider the following chemical reaction: 

In this reaction, Nitrogen reacts with Hydrogen to produce Ammonia. To be more precise, one mole of Nitrogen reacts with two moles of Hydrogen to produce two moles of Ammonia. Since your chemical reaction equation is balanced, we can use stoichiometry to compare quantities of reactants and products to understand the relationship between compounds in a chemical reaction. 

 It is important to understand that to compare the quantities in the above reaction, we use moles and not mass. Moles are units that describe the number of things, like 6.022 x 10²³. Consider the following response with two apples to two pies.

In this case we have four apples and two pies to relate the apples to pies through a ratio of quantities or similarly with other ingredients. It is incorrect to only use the mass of ingredients and pies and the equation above to determine how many apples are required or how many pies will be made. The first step requires calculating the weights of one apple or one pie. In a chemical reaction we refer to quantity ratios as a mole ratio and the mass per mole as Molar mass.  

Now that we understand moles and molar mass, we can determine the amount of reactants we need for a reaction or predict how much product we will achieve given an amount of reactants.  

Let’s tackle one introductory stoichiometry problem and come up with a game plan on how to approach these types of questions.  

How many moles of chlorine, Cl₂, is required to react with 10.0 g of sodium metal Na to produce NaCl? Molar mass of Cl: 35.453 g/mol, Molar mass of Na: 22.990 g/mol

Start by writing the balanced equation and determine what information you know and don’t know!  

Here's what we know:

• Cl₂ + 2Na --> 2NaCl
  

• 10.0 g of Na 

• The molar mass of  Cl₂ and Na

 Here's what is unknown: 

• Moles of Cl₂ required 

Use the information about the given amount of Na to determine its moles required.  

Use mole ratios from the balanced equation to determine the moles of Cl₂ required. 

Your final answer is 0.217 mol of Cl₂. Consider what this final result can also help you determine! For example, with the final answer you can also find the grams of Cl₂ required since its molar mass was given.

With this example, it is necessary to write down a correct balanced equation, find information such as molar mass from the periodic table, and apply the molar ratios. Thus, using stoichiometry will allow you to compare the quantities in a chemical reaction to determine reactant needs or predict product amounts.