Unit 7: Stoichiometry

A set of review materials for Unit 7: Stoichiometry

The Mole

A mole (mol) is a unit of scale used to count atoms. Originally developed by Amadeo Avogadro, 1 mol = 6.022 x 10^23 particles

We need this because when we are talking about chemical reactions, the coefficients we make in our balanced reaction are ratios between the numbers of molecules of different compounds. But counting molecules is almost impossible. Instead we use moles as a way of counting the enormous number of atoms needed to have an amount that we can physically handle.

When doing the math, use Avogadro's Number as a conversion factor.

Check out this video if you need more help:

Converting between Moles and Particles

Molar Mass

Molar mass is the relationship between the number of atoms in a sample and the mass of the sample. Simply put, it is the mass of a mole. To make the math super easy, the molar mass of an element is the average atomic mass of that element as found on the periodic table. The unit for molar mass is grams per mole (g/mol).

To calculate the molar mass of a compound, you simply will need to add up the average atomic mass of each atom present in the compound.

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How to Calculate Molar Mass

Gram <=> Mole Conversions

When converting between grams and moles, use the unit for molar mass (g/mol) to help you! This will tell you how many moles you have in a specific mass sample or how many grams you would need to have a specific number of moles. Here are the conversions in a nutshell:

  • Grams => moles, divide by the molar mass.
  • Moles => grams, multiply by the molar mass.

You can also combine this with mole to particle and particle to mole conversions!

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Calculating Moles from Grams (Mass to Moles)

Percent Composition

Percent composition is a way of comparing how much of a sample is a given element.

Originally, we had to do very careful experiments to determine how much of a sample of copper oxide was copper and how much was oxygen. When we did, we noticed that the percentages were always either 89% copper and 11% oxygen or 80% copper and 20% oxygen. After a lot of careful math, you can determine that these percentages correspond to the two different compounds formed between copper and oxygen: Copper (I) Oxide and Copper (II) Oxide. These percentages are how we originally developed chemical formulas and determined the subscripts in all compounds.

Calculating the percentage composition from the formulas is not too challenging if you remember that the math for any percent is (part/whole) x 100. In this instance, the 'part' is the part of the molar mass that is element A and the 'whole' is the molar mass of the whole thing

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Percent Composition By Mass

Empirical Formula

An empirical formula is the simplest ratio of elements in a compound. Most formulas for ionic and many covalent compounds are already in their empirical formulas.

Empirical formulas can be calculated from a percent composition by following these steps:

  • Assume a 100 g sample (change % to g)
  • Convert the grams for each element to moles (divide by molar mass)
  • Reduce the ratio (divide by the smallest number)
  • optional: scale to whole numbers (1/2 => multiply by 2. 1/3 or 2/3 => multiply by 3. 1/4 or 3/4 => multiply by 4)
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Empirical Formula and Molecular Formula Introduction

Molecular Formula

A molecular formulas is any formula that is not in it's simplest ratio. The relationship between a molecular formula and an empirical formula is always some integer multiple.

Empirical x n = Molecular

As such, the relationship between the molar mass of the empirical formula and the molecular formula will always be the exact same integer.

Empirical Mass x n = Molecular Mass

Molecular formulas can be calculated from the empirical formulas if you know the molar mass of the molecular formula.

Calculating Molecular Formula from Empirical Formula
Molecular and Empirical Forumlas from Percent Composition

The Mole Ratio (Using a Chemical Reaction)

The coefficients in a chemical reaction can be turned into a mole ratio. Use this when you want to translate between moles of compound A to moles of compound B. Always be sure to put the compound you are trying to cancel out in the denominator and the compound you are trying to examine in the numerator.
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Mole Ratio Practice Problems

Putting It All Together (Stoichiometry)

Stoichiometry is the math needed to convert between grams of two different compounds in a chemical reaction. It can answer questions like:

  • How many grams of hydrogen peroxide are needed to make 15 grams of water?
  • How many grams of silver are made when 16 grams of copper reacts?
  • How many grams of oxygen are needed to react with 20 grams of hydrogen?

At the most, Stoichiometry is a three step process:

  1. Grams of compound A to moles of compound A (divide by molar mass)
  2. Moles of compound A to moles of compound B (multiply by the mole ratio)
  3. Moles of compound B to grams of compound B (multiply by the molar mass)
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Limiting Reactant

Here is my recipe for a sandwich:

  • 2 slices of bread
  • 2 slices of meat
  • 1 slice of cheese
  • 3 slice of tomato
How many sandwiches can I make from a bag of bread , 1 pound of meat, half a pound of cheese and five tomatoes?
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Stoichiometry: Limiting Reagent

Percent Yield

Just because the math predicts that we should make a certain amount of product does not mean we will produce the correct amount. This is due to a variety of issues including contamination and slow reaction rates.

As such, we are going to use percent yield to compare the amount that we actually made to the amount that we predicted that we should have made using this formula.

(Actual / Theoretical) x 100 = Percent Yield

Limiting Reactants and Percent Yield