STOICHIOMETRY NOTESChemistry 2013-2014

• Stoichiometry is the calculation of quantities in chemical equations.

• Stoichiometry can be used to predict the amount of product that will form in a reaction. It can also be used to determine the amount of each product you need to use, if you want to get a certain amount of a product.

• The “heart” of stoichiometry is the mole ratio, which is the ratio of the coefficients of two things in a chemical reaction. For example, if you look at the equation in the box for the formation of water from hydrogen gas and oxygen gas, the mole ratio of hydrogen to water is 2:2 or 1:1.

What is the mole ratio of H2 to O2? ______Or O2 to H2? _______What is the mole ratio of O2 to H2O? ______Or H2O to O2? _______

Besides mole ratios, we will also need our old conversion factors.

• 1 mole = GFM

• 1 mole = 6.02 x 1023 atoms (elements), molecules (molecular compounds), or formula units (ionic compounds).

• 1 mole = 22.4 L at STP

• One of the governing laws of Chemistry is the Law of Conservation of Mass, which states that matter cannot be created or destroyed. Another way of saying this is that atoms cannot be created or destroyed (though they can be changed, combined, or split through nuclear reactions).

Are these conserved in a chemical reaction?• Mass Yes No• Number of atoms Yes No• Moles Yes No• Molecules Yes No• Volume Yes No

• Before you start any stoichiometry problem, you need to write a balanced equation.

• Never assume an equation is balanced—always check for yourself. If the equation is not balanced, or if it’s balanced incorrectly, you will get the wrong coefficients, which will give you the wrong mole ratio, which will give you the wrong answer.

• For example, if you used the equation in the top-right corner of this page as-is, you would think the mole ratio of nitrogen to hydrogen is 1:1. If you used this mole ratio in a stoichiometry problem, you would get a wrong answer.

• Balance the equation.• What is the real mole ratio of nitrogen to hydrogen?

• There are several types of stoichiometry problems. Let’s start off simple with mole-mole calculations. In mole-mole calculations, you need to convert from moles of one substance to moles of another. This type of problem only uses one conversion factor, the mole ratio.

How many moles of aluminum are needed to form 2.3 moles of aluminum oxide?

____ Al + ____ O2 ____ Al2O3

 

____ Al + ____ O2 ____ Al2O3

How many moles of oxygen are required to react completely with 0.84 mol of Al?

• Mole-mole calculations are the simplest because they only use one conversion factor. Here are other types of stoichiometry problems:

• Mass-mole or mole-mass calculations are two-step problems that use GFM and mole ratio

• Mass-mass calculations are three-step problems that use GFM and mole ratio

• Mole-volume calculations are two-step problems that use 1 mole = 22.4 L and mole ratio

• Volume-volume calculations are three-step problems that use 1 mole = 22.4 L and mole ratio• However, if you convert from liters to moles, then back to liters, you’ll

notice that the 1 mole = 22.4 L conversion factor gets canceled. There is a shortcut for volume-volume calculations: use the mole ratio as a “volume ratio” to turn these into one-step problems.

• Volume-mass or mass-volume calculations are three-step problems that use GFM, 1 mole = 22.4 L, and mole ratio

• Sometimes problems begin, end, or begin AND end with “atoms”, “molecules”, “formula units”, or “particles”. In this case, you will need to use Avogadro’s number, 1 mole = 6.02 x 1023 atoms (elements), molecules (molecular compounds), or formula units (ionic compounds).

Remember that the mole ratio is the “heart” of stoichiometry. If your given is not in moles, you will need to convert it to moles (this will be the first step of the problem). Use the mole ratio to figure out how many moles of the substance you want are present, and then change those moles or grams, liters, etc. according to the problem statement.

Example 1

Calculate the number of grams of solid sodium oxide that will be produced when 3.60 moles of solid Na react with O2.

 

Balance the equation: _____ Na + _____ O2 _____ Na2O

What kind of problem is this? ________________________

Example 1 continued

Calculate the number of grams of solid sodium oxide that will be produced when 3.60 moles of solid Na react with O2.

 4 Na + 1 O2 2 Na2O

Set up the problem and solve. Always show your work.

Example 2

How many grams of oxygen are required to “burn” 13.0 g of C2H2?

Balance the equation

_____ C2H2 + _____O2 _____ CO2 + _____ H2O

What kind of problem is this? _______________________

Example 2 continued

How many grams of oxygen are required to “burn” 13.0 g of C2H2?

2 C2H2 + 5 O2 4 CO2 + 2 H2O

Set up the problem and solve. Always show your work.

Example 3

How many grams of CO2 and grams of H2O are produced when 13.0 g of C2H2 reacts with oxygen?

2 C2H2 + 5 O2 4 CO2 + 2 H2O

What kind of problem is this? _______________________

Example 3 Continued

How many grams of CO2 and grams of H2O are produced when 13.0 g of C2H2 reacts with oxygen?

2 C2H2 + 5 O2 4 CO2 + 2 H2O

Set up the problem and solve. Always show your work.

 

 

 

Use answers from Example 2 and Example 3 to show that this equation obeys the law of conservation of mass.

Example 4

How many grams of SnF2 can be made by reacting 7.42 x 1024 molecules of HF with tin?

Balance the equation

_____ HF + _____ Sn _____ SnF2 + _____ H2

What conversion factors do you need to solve this problem? ________________________

Example 4 continued

How many grams of SnF2 can be made by reacting 7.42 x 1024 molecules of HF with tin?

2 HF + 1 Sn 1 SnF2 + 1 H2

Set up the problem and solve. Always show your work.

Example 5

How many liters of ammonia, NH3, are produced by the reaction of hydrogen gas with 80.0 L of nitrogen gas at STP?

Balance the equation

_____ N2 + _____ H2 _____ NH3

What conversion factors do you need to solve this problem? ________________________

Example 5 continued

How many liters of ammonia, NH3, are produced by the reaction of hydrogen gas with 80.0 L of nitrogen gas at STP?

1 N2 + 3 H2 2 NH3

Set up the problem and solve. Always show your work.

Note: there are two ways to solve this problem—in three steps, or in one.

Example 6

How many molecules of hydrogen gas are needed to produce 1.2 L of ammonia at STP?

1 N2 + 3 H2 2 NH3

What conversion factors do you need to solve this problem? ________________________

Set up the problem and solve. Always show your work.