Back To CourseChemistry 101: General Chemistry
14 chapters | 132 lessons | 11 flashcard sets
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Nikki has a master's degree in teaching chemistry and has taught high school chemistry, biology and astronomy.
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.
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.
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:
Once we have this information, we multiply the moles of our given by our mole ratio between our desired substance and given substance. This will give us the number of moles of our desired substance:
(moles of given substance / 1) * (moles of desired substance / moles of given substance) = moles of desired substance
Like in the cookie problem, some of our units will cancel. In this case, it will be moles of given substance. Back to our water problem - I want to know how many moles of O2 I will need to make 4.5 moles of water. My balanced equation is:
2 H2 + O2 --> 2 H2 O
I gather my necessary information: I have 4.5 moles of water as my given substance and moles of O2 as my desired substance. The mole ratio between my desired substance (O2) and my given (H2 O) is 1 mole O2 / 2 moles H2 O. I put this info into my equation:
(4.5 moles H2 O/ 1) * (1 mole O2 / 2 moles H2 O) = moles of O2 needed.
The 'moles of H2 O' cross out, so I'm left with moles O2 as my units. I do a little math:
(4.5 * 1 mole O2) / (1 * 2) = moles of O2 needed
2.25 moles O2 = moles of O2 needed
I have the following balanced reaction equation:
2 Al2 O3 --> 4Al + 3 O2
Let's say I'm curious about how many moles of O2 I'll make from the decomposition of 10 moles of Al2 O3 . To begin, I gather my three pieces of necessary information:
10 moles of Al2 O3 is my given substance; moles O2 is my desired substance. The mole ratio between desired (O2) and given (Al2 O3 ) is 3 moles O2:2 moles Al2 O3 . Now, I input this info into my equation:
(10 moles Al2 O3 / 1) * (3 moles O2 / 2 moles Al2 O3 ) = moles O2
'Moles Al2 O3 ' will cancel out.
(10 * 3 moles O2) / (1 * 2) = 15 moles O2. 15 moles O2 will be produced.
Let's do one more. I'm performing the following reaction in lab:
4 NH3 + 6NO --> 5 N2 + 6 H2 O
I want to produce 100 moles of N2. How many moles of NO do I need to start with? I gather my three pieces of information: 100 moles of N2 is my given substance; moles NO is my desired substance; the mole ratio between desired and given is 6 moles NO:5 moles N2 . Next, I'll input information into my equation:
(100 moles N2 / 1) * (6 moles NO / 5 moles N2) = moles of NO needed
The 'moles of N2 ' cross out.
(100 * 6 moles NO) / (5 * 1) = 120 moles of NO needed
120 moles of NO = moles of NO needed
Stoichiometry is the study of chemical quantities consumed or produced in a chemical reaction. Stoichiometry is performed in terms of moles. A mole is a chemical counting unit, such that 1 mole = 6.022*1023 particles. Stoichiometry also requires the use of balanced equations. From the balanced equation we can get the mole ratio. The mole ratio is the ratio of moles of one substance to the moles of another substance in a balanced equation. Use of mole ratios allows us to convert from one chemical substance to another.
To determine the mole ratio for two substances in a chemical reaction, look at the coefficients in front of each species in the balanced chemical equation. When performing mole-to-mole calculations, you'll need three pieces of information: moles and identity of given substance, identity of desired substance, and mole ratio between given substance and desired substance. Plug this information into your equation and solve mole-to-mole problems:
Once you've finished with this lesson, you should have the ability to:
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Back To CourseChemistry 101: General Chemistry
14 chapters | 132 lessons | 11 flashcard sets