**How much equipment do you need for an experiment?**

You are in charge of setting out the lab equipment for a chemistry experiment. If you have twenty students in the lab (and they will be working in teams of two) and the experiment calls for three beakers and two test tubes, how much glassware do you need to set out? Figuring this out involves a type of balanced equation and the sort of calculations you would do for a chemical reaction.

### Everyday Stoichiometry

You have learned about chemical equations and the techniques used in order to balance them. Chemists use balanced equations to allow them to manipulate chemical reactions in a quantitative manner.

You may have used a recipe to make a batch of cookies. If the recipe quantities make five dozen cookies, and you only need two and one half dozen, the recipe can be scaled down. For example:

Chocolate Chip Cookies (makes 5 dozen)

1 1/3 cup butter, 1 cup sugar, 1 cup brown sugar, 2 eggs, 2 teaspoons vanilla, 3 cups flour, 1 teaspoon soda, 1 teaspoon salt, 1 bag chocolate chips

To scale the recipe down and make only 2 1/2 dozen cookies, the amount of ingredients must be cut in half:

2/3 cup butter, 1/2 cup sugar, 1/2 cup brown sugar, 1 egg, 1 teaspoon vanilla, 1 1/2 cup flour, 1/2 teaspoon soda, 1/2 teaspoon salt, 1/2 bag chocolate chips

Before we look at a chemical reaction, let’s consider the equation for the ideal ham sandwich.

Our ham sandwich is composed of 2 slices of ham (H), a slice of cheese (C), a slice of tomato (T), 5 pickles (P), and 2 slices of bread (B). The equation for our sandwich is shown below:

\begin{align*}2\text{H}+\text{C}+\text{T}+5\text{P}+2\text{B} \rightarrow \text{H}_2\text{CTP}_5\text{B}_2\end{align*}

Now let us suppose that you are having some friends over and need to make five ham sandwiches. How much of each sandwich ingredient do you need? You would take the number of each ingredient required for one sandwich (its coefficient in the above equation) and multiply by five. Using ham and cheese as examples and using a conversion factor, we can write:

\begin{align*}5 \text{H}_2\text{CTP}_5\text{B}_2 \times \frac{2 \ \text{H}}{1 \ \text{H}_2 \text{CTP}_5\text{B}_2} &= 10 \ \text{H}\\ 5 \text{H}_2\text{CTP}_5\text{B}_2 \times \frac{1 \ \text{C}}{1 \ \text{H}_2\text{CTP}_5\text{B}_2} &= 5 \ \text{C}\end{align*}

The conversion factors contain the coefficient of each specific ingredient as the numerator and the formula of a sandwich as the denominator. The result is what you would expect. In order to make five ham sandwiches, you would need 10 slices of ham and 5 slices of cheese.

In the same manner, chemical equations can be scaled down or up to make a quantity that is different than in a balanced equation. This type of calculation demonstrates the use of stoichiometry. **Stoichiometry** is making relationships in the calculation of the amount of substances in a chemical reaction from the balanced equation. The sample problem below is another stoichiometry problem involving ingredients of the ideal ham sandwich.

#### Sample Problem: Ham Sandwich Stoichiometry

Kim looks in the refrigerator and finds that she has 8 slices of ham. In order to make as many sandwiches as possible, how many pickles does she need? Use the equation above.

*Step 1: List the known quantities and plan the problem.*

Known

- have 8 ham slices (H)
- 2 H = 5 P (conversion factor: for two slices of ham, Kim needs 5 pickles)

Unknown

- How many pickles (P) needed?

The coefficients for the two reactants (ingredients) are used to make a conversion factor between ham slices and pickles.

*Step 2: Solve.*

*\begin{align*}8 \ \text{H} \times \frac{5 \ \text{P}}{2 \ \text{H}}=20 \ \text{P}\end{align*}*

Since 5 pickles combine with 2 ham slices in each sandwich, 20 pickles are needed to fully combine with 8 ham slices.

*Step 3: Think about your result.*

The 8 ham slices will make 4 ham sandwiches. With 5 pickles per sandwich, the 20 pickles are used in the 4 sandwiches.

#### Summary

- Stoichiometry is making relationships in a balanced chemical equation..

#### Practice

*Questions*

Use the link below to answer the following questions:

http://www.chem4kids.com/files/react_stoichio.html

- What does stoichiometry help you figure out?
- What are all reactions dependent upon?
- If I have ten hydrogen molecules and three oxygen molecules, how many molecules of water can I make?
- What will be left over and how much?

#### Review

*Questions*

- I don’t like pickles. What would my ideal ham sandwich be?
- How does that change the equation?
- Will this change affect the amounts of other materials?