Stoichiometry is the study of how the amounts of reactants in a chemical reaction relate to the amount of products produced in the reaction.

Be sure to check out the worksheet at the bottom of this page for extra stoichiometry practice ## The Basics

The basis of stoichiometry is the chemical reaction. If you are not familiar with the process of balancing chemical reactions, click the button to the right.

The number in front of each reactant and product in a chemical equation creates a ratio between all the reactants and products in a reaction. In the methane combustion reaction above, for example, you need 1 molecule of methane for every 2 molecules of oxygen. The result will be 1 molecule of carbon dioxide and 2 molecules of water.

When chemists design chemical reactions, it is nearly impossible to isolate an exact number of molecules of each atom. This is why they use moles. 1 mole is always 6.022X10^23 molecules. This means that you can replace the proportionality of molecules with the same proportionality of moles. In the reaction above, for every mole of methane, you need 2 moles of oxygen to produce 1 mole of carbon dioxide and 2 moles of water.

Stoichiometry is the process finding the right quantity of a compound to make the reaction function in a desired way. This involves finding the correct number of moles to complete an equation, then determining how much mass of that substance would make that amount of moles. Let's try an example.

#### Review

If this section is confusing, click the button below to access our page on balancing reactions.

## Limiting Reactant

#### Identifying a limiting reactant problem

In the example above, there is an excess of Sodium carbonate, so all of the HCl is able to react. But what happens if there is not enough of one product for all of the other product to react? In this situation, we have a limiting reactant. A clue that you are dealing with a limiting reactant problem is if the question states a mass of both (or all) of the reactants.

#### Finding the Limiting Reactant

When tasked with finding the limiting reactant, you will be given the mass of both reactants. First, convert the given masses to moles by dividing by the molar mass. Then, divide both of the moles of the reactants by the number of molecules needed to complete the reaction (the number in front of the chemical formula in the reaction equation). The reactant with the smaller value is the limiting reactant.

#### Stoichiometry Problem With Limiting Reactant

Stoichiometry questions with a limiting reactant are a staple of chemistry assessments. The first step to solve a stoichiometry question with a limiting reactant is to first be confident in solving a typical stoichiometry question like the example above.  However, the most important aspect to remember is that you must use the moles of the limiting reactant in your calculations as all the limiting reactant used up while there will be an excess of the other reactant.

## Empirical and molecular formulas

#### Empirical Formula The empirical formula  of a compound shows the ratios between all of the elements in their most reduced form. Follow the steps of the example above to write an empirical formula.

#### Empirical Formula The molecular formula of a compound shows the actual, specific formula of a compound based on the ratios of its elements and its overall molar mass.

## Percent Yield Percent yield allows us to calculate how much of a product actually came out of an experimental reaction by comparing the physical results to a theoretical calculation.

## Stoichiometry Worksheet

Stoich worksheet and answer key (pdf)