What Is Stoichiometry?

What Is Stoichiometry?

Read Time: 4 minutes

What is stoichiometry? It sounds like an intimidating science subject. However, it’s a simple concept that’s critically important to chemistry.

Stoichiometry looks at the relationship between reactants and products in chemical reactions. Quantitatively, it’s the relation between the number of moles — and therefore, mass.

Reactions must be balanced. It must have the same number of atoms in the products as in the reactants. If the reaction is not balanced, you can’t derive any information about the relationship.

Stoichiometry can help you determine how much of a reactant you need to create an end product. It can also help you figure out how much you started with.

What Is a Chemical Reaction?

In simple terms, a chemical reaction occurs when one set of chemicals transform into another. The bonds between atoms change, able to break and rearrange to form new ones. The result features entirely different properties.

Think of it like making brownies. You start with everyday ingredients like flour, cocoa and eggs. However, when you combine them, they change to form a new substance — a delicious treat.

What is stoichiometry, and how does it relate to chemical reactions? To become an expert, you’ll need to understand a few key concepts, including:

  • Balancing chemical equations
  • Finding and calculating molar mass
  • Calculating mole ratios

What Is a Balanced Equation?

To understand how much of a reactant you need in stoichiometry, you need to balance the chemical equation. According to the Law of Conservation of Mass, discovered by Antoine Lavoisier in 1789, atoms are not destroyed or created during a reaction.

Because of this, a balanced equation must have the same number of atoms on both the right and left sides.

For example, a balance equation might look like:

2H2 + O2 = 2H2O

You will see an equal number of hydrogen and oxygen atoms on each side of the equation.

How to Calculate Molar Mass

Once the equation is balanced, you can move onto the next step, which is converting the mass values of the reactants into moles. What is a mole? In chemistry, it’s not a small rodent. Instead, it’s a unit used to measure the amount of a substance.

A mole allows chemists to measure out equal amounts of elements. To calculate the molar mass of a reactant, you must add all of the atomic masses in the molecule.

For example, say you want to calculate the molar mass of H2O. You would need to find the atomic mass of a hydrogen atom. Using the periodic table, one atom of hydrogen is 1.008. Two atoms of hydrogen has a mass of double that amount, 2.016. Now it’s time to look at oxygen, which has an atomic mass of 15.999. Together, 15.999 + 2.016 = 18.015. Therefore, the molar mass of water is 18.015 grams per mole.

How to Calculate Mole Ratios

The next step to solve an equation is to calculate mole ratios. This number is the ratio between the moles in each reactant in a chemical reaction.

Again, consider the equation: 2H2 + O2 = 2H2O. In this example, the mole ratio is 1:2. For every two moles of H2, there is one mole of O2.

Let’s imagine we’re trying to determine how much H2 we need to create a product. We know we have 25 grams of O2. To determine the answer, we can use stoichiometry. First, we must convert the grams to moles as we did earlier, by calculating molar mass.

You will find 15.999 grams per mole of oxygen. Therefore, O2 has a molar mass of 31.998 grams. In this example, we only have 25 grams of O2, making it necessary to divide 25 by 31.998, which equals .7813 moles. Remember, for every mole of O2, there are two moles of H2. Take the previous number — .7813 moles — and multiply it by two, which is 1.57 moles.

We know the molar mass of 2H2 is 4.032. To find out how many grams we need, take the 1.57 moles, and multiply it by 4.032. As a result, we need 6.29 grams of H2.

Why Is Stoichiometry Important?

What is stoichiometry? Now you know! Yet how does it apply to real life?

This science is useful across a variety of fields. You can use it in the kitchen while cooking. Say you’re almost out of a particular ingredient. You can use stoichiometry to determine how much of every other food you’ll need. Next time you want to make brownies and don’t have enough milk, don’t despair. Instead, use this concept to create a tasty treat.

A farmer can use stoichiometry to determine how much fertilizer to use. A traveler can figure out how fast they need to go to get somewhere by a specific deadline. You can also use it to make conversions between systems, such as Celsius and Fahrenheit.

Now that you understand the basics of stoichiometry, you can delve into more profound concepts!

What Is Stoichiometry?
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Category: Chemistry


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Article by: Megan Ray Nichols

Megan Ray Nichols is a freelance science writer and science enthusiast. Her favorite subjects include astronomy and the environment. Megan is also a regular contributor to The Naked Scientists, Thomas Insights, and Real Clear Science. When she isn't writing, Megan loves watching movies, hiking, and stargazing.