Stoichiometry

Underlying concepts

What is a Mole?

Because atoms are so small, for convenience, a quantity was created to represent the number of particles in a sample of a given substance. This unit is called the mole, and is represented by Avogadro's number, 6.022x10²³ atoms.

What is Molar Mass and how is it determined?

Molar mass is the amount in grams of one mole, or 6.022x10²³ atoms, of a given substance. It is determined by looking at the periodic table. In each element square, there is a number across the bottom; it is the element's molar mass. For example, if one were to look at the element carbon, C, on the periodic table, the number at the bottom of carbon's square is 12.0001. Thus, carbon's molar mass is approximately 12 grams. The same thing applies to compounds. To determine the molar mass of calcium carbonate, or CaCO₃, each element in the molecule would have to be looked up on the periodic table. One mole of calcium weighs 40.1g; one mole of carbon weighs 12g; and one mole of oxygen weighs 16g. However, because there are three oxygen atoms per molecule of CaCO₃, the molar mass of oxygen must be multiplied by three in order to account for all three molecules. Then these three weights must be added together to come up with a final total molar mass for the entire molecule. For CaCO₃, the molar ends up to be 100.1g.

What is stoichiometry and what is it used for?

Stoichiometry is a depiction of the mass and amount relationships between the products and reactants in a chemical reaction. It is useful to predict how much of a substance will be involved in a reaction. It is also very useful in that it allows conversion from one unit to a variety of others. For example, using stoichiometry, it can easily be determined how many atoms of CO₂ are in 2.5L of CO₂

Problem Solving Steps for Stoichiometry Problems

List what you know

1. Read the problem carefully

2. Organize the given information in a list or a table

3. Identify what needs to be found and write down the units for the answer

4. If there is a reaction involved, write a balanced equation for it

5. List necessary conversion factors such as molar masses, mole ratios, and unit conversions

Set up the problem

1. Analyze what needs to be done to get the answer, and determine what, if any, necessary information for obtaining the answer is missing

2. Write formulas for all substances being worked with and determine their molar masses

3. Identify which given can be used as a starting point. Write an equals sign to the right of the starting point and then a question mark with the units of the answer. Fill in the units and the conversion factors and the numerical values necessary to convert from what is given in the problem to what is sought in the answer, and be sure that you are using the correct units

4. Nearly all stoichiometry problems require the answer to be in moles, so use molar masses to convert the answer to moles

5. If it is necessary to change from moles of one substance to moles of another substance, use the mole ratios derived from the balanced equation, and remember that it is not always 1:1

6. Check unit cancellations before performing calculations, to do this, cancel the units diagonally, from upper left to lower right. For example 5 kg x 1000 g = 5000 g. Cancel out the units,  the kgs cancel each other out. 

                          1 kg

Calculate and verify

1. Begin by working the setup from above

2. Round off and make sure the significant figures of the answer are correct

3. Make sure the units are correct

4. Verify the answer by estimating

5. Make sure the answer is reasonable. If it is not, go back and recheck work

Sample Problems and Solutions

1) How many moles of NaCl are in 10.0g NaCl?

Given: 10.0g NaCl

Molar mass of NaCl: 23g Na+ 35.45g Cl = 58.45g/mol NaCl

Setup: 10.0g NaCl x 1mol NaCl    = ?mol NaCl

                                 58.45g NaCl 

Solution: .171mol NaCl

2) The human body needs at least 1.03x10^-2 mol O₂ every minute. If all this oxygen is used for the cellular respiration reaction which oxidizes glucose to carbon dioxide and water, what mass of oxygen, in grams, does the human body consume each minute?

Write the balanced equation:

C₆H₁₂O_6(aq) + 6O_2(aq) = 6CO_2(aq) + 6H₂O_(l)

Givens: Amount of O₂ used each minute: 1.03x10^-2 mol O₂

Mole ratio of glucose to oxygen: 1 mol C₆H₁₂O₆ : 6 mol O₂

Molar mass of glucose: 180.18g/mol C₆H₁₂O₆

Because oxygen is given in a mole amount, the molar mass of oxygen is unnecessary

Setup: 1.03x10^-2 mol O₂ x 1 mol C₆H₁₂O₆ x 180.18g C₆H₁₂O₆  = ? g C₆H₁₂O₆

                                                             6 mol O₂                    1 mol C₆H₁₂O₆

Solution: .309g C₆H₁₂O₆

Click here for some sample multiple choice problems

Click here for a site that's very helpful!

Created by Priya and Kristen Class of 2002