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Mole-to-Mole Ratios and Calculations of a Chemical Equation

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  • 0:01 Stoichiometry
  • 2:01 Mole Ratios
  • 4:20 Mole-to-Mole Calculations
  • 6:59 Examples
  • 9:05 Lesson Summary
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Lesson Transcript
Instructor: Elizabeth (Nikki) Wyman

Nikki has a master's degree in teaching chemistry and has taught high school chemistry, biology and astronomy.

Learn what a mole ratio is and how to determine and write the mole ratio relating two substances in a chemical equation in this video lesson. Also, learn to make mole-to-mole calculations and solve problems involving moles of substances.

Stoichiometry

Pretend you want to make chocolate chip cookies. You have a great recipe handed down from your grandmother that calls for two cups of chocolate chips, but you only have one cup of chocolate chips in the house. It's raining outside, and you don't feel like going to the store. So, what do you do? Do you make the cookies with half the chocolate chips the recipe calls for? No way! Who wants to eat cookies with only half the chocolate?

Instead, you determine the ratio of chocolate chips on hand to amount needed, which is 1:2. Then, you adjust the ratio of all the other ingredients in the recipe. Essentially, you have just performed stoichiometry, one of the fundamental aspects of chemistry. Stoichiometry is a word derived from two Greek words: 'stoicheon' meaning element, and 'metron,' meaning measure. This is pretty cool because stoichiometry is essentially the measurement of elements, or the study of chemical quantities consumed or produced in a chemical reaction.

Performing stoichiometry involves the use of a special chemical counting unit: the mole. Just to review for a moment, a mole isn't an animal. Well, it is, but not in chemistry. In chemistry, a mole is a unit of measurement, such that one mole of a substance contains 6.022*1023 particles.

In chemistry, particles can be atoms, molecules, or compounds. Conveniently, one mole of a substance has a mass that is equal to its atomic mass expressed in grams. This relationship is known as molar mass. For example, one atom of carbon has a mass of 12.011 amu, one mole of carbon has a mass of 12.011 grams.

When we do stoichiometry, we always want to speak about chemicals in terms of how many moles are present. The essence of stoichiometry involves comparing how many moles of chemicals are present. We may be simply comparing the number of moles of each reactant needed, or the number moles reactant to number of moles product.

Mole Ratios

Hopefully, the process of balancing a chemical reaction is still fresh in your mind. But, do you remember why we balance equations? We balance them to obey the law of conservation of mass, which states that matter cannot be created or destroyed. Balancing equations has another benefit, though. A balanced equation sets us up to perform flawless stoichiometry. Let's take the balanced equation for the formation of water:

2 H2+ O2--> 2 H2 O

Notice that this equation contains the lowest whole number coefficients possible. From this, we can garner exactly how many moles of hydrogen are needed to react with one mole of oxygen. In stoichiometry, we shift our unit from molecule to mole.

According to this equation, we need two moles of hydrogen to react with one mole of oxygen. This is called the mole ratio. It is defined as the ratio of moles of one substance to the moles of another substance in a balanced equation.

To determine the mole ratio between two substances, all you need to do is look at the balanced equation for the coefficients in front of the substances you are interested in. Let this be your guiding mantra for doing stoichiometry problems!

The balanced equation for water has several mole ratios in it. There are two moles of H2 for every one mole O2. We will write this ratio as:

2 moles H2 / 1 mole O2

There are two moles H2 for every 2 moles H2 O:

2 moles H2 / 2 moles H2 O

There is one mole O2 for every 2 moles H2 O:

1 mole O2 / 2 moles H2 O

Why do we care about mole ratios so much? Mole ratios are the central step in performing stoichiometry because they allow us to convert moles of one substance to moles of another substance.

Another balanced equation is the making of ammonia:

N2 + 3 H2 --> 2 NH3

Pause the video right now, and see if you can identify three mole ratios in this equation. Remember, look at the coefficients in the balanced equation! Here are three mole ratios:

1 mole N2 / 3 moles H2

3 moles H2 / 2 moles NH3

1 mole N2 / 2 moles NH3

If you express any of these ratios upside down from what was shown, like 3 moles H2 / 1 mole N2, you are still right! Nice work.

Mole-to-Mole Calculations

Let's go back to the cookie analogy. Pretend that I now have plenty of ingredients. I need to make 100 cookies, but the recipe only makes 25. How do I figure out how many cups of chocolate chips I need? I'm not going to count the individual chocolate chips, just like I don't want to count individual atoms. I'm going to measure it by the cup. My ratio is 2 cups chocolate chips: 25 cookies. I want 100 cookies, so I set up the equation as:

(100 cookies/1) * (2 cups of chips / 25 cookies) = cups of chips needed

The 'cookies' crosses out, leaving me with cups of chips as my unit - exactly what I want. I do some math (100 * 2) / (25 * 1), and find that 8 cups of chips = the cups of chips needed. Now we're just going to apply the same idea to substances involved in a chemical reaction rather than a recipe.

So, what happens if I want to make 4.5 moles of water? How many moles of oxygen do I need? This is a typical mole-to-mole calculation. When performing these calculations, you will need three pieces of information:

  • Moles and identity of given substance
  • Identity of desired substance
  • Mole ratio between given substance and desired substance. Remember, mole ratio comes from the balanced chemical equation!

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