Welcome to Stoichiometry Made Easy with Ketzbook
In this video, we are going to learn “What is stoichiometry?,” and “How to
solve one step stoichiometry problems.”
But, before we begin, let’s start with a question.
Which of the following always contains the same number of atoms?
a) 1 gram of atoms b) 1 mole of atoms
c) 1 liter of atoms
The correct answer is “b”, 1 mole of atoms, which is equal to 600 billion trillion atoms.
Remember that just like a dozen is twelve things, a mole is simply a very large number
of things. And because the mole refers to the number of things, we will need to use
the mole extensively when we do stoichiometry.
So, what in the world is stoichiometry? The word comes from two Greek roots, “stoichion,”
which means element, and “metron,” which means measure. Basically, stoichiometry is
a scientist’s way of measuring elements and other chemicals in a chemical reaction.
It’s a lot like the way a chef measures the ingredients of a recipe. In fact, chemistry
is a lot like cooking. Imagine for a second that you’re home alone and craving some
grilled cheese sandwiches, but...you don’t know how to make one. So you call up your
friend and say, “hey, man, how do you make a grilled cheese sandwich?” And he says,
“dude, chill...it’s real easy, just take some bread, put some cheese in the middle,
heat it up, and you got yourself some grilled cheese.” “How much bread? How much cheese?”
you ask. “Two slices of bread, and one slice of cheese is all you need to make one sandwich.”
So there you go, you’ve got your recipe. In chemistry, the recipe is the balanced chemical
equation. So where does stoichiometry come in? Let’s suppose you are super hungry and
you want to make 7 grilled cheese sandwiches. You need to know how much of each ingredient
to use. Let’s start with the first ingredient. How many slices of bread do you need to make
7 grilled cheese sandwiches?
You can probably solve this in your head, but please follow the logic of my method.
In order to solve this problem, we are first going to write down the quantity that we know,
7 grilled cheese sandwiches. Next, we are going to multiply this by the ratio of bread
to sandwiches. You need two slices of bread to make one sandwich. Where did I get those
numbers? The recipe or balanced equation tells us the ratio of the ingredients and products.
Bread and sandwiches are in a 2 : 1 ratio. We put the 2 slices of bread on top because
that is what we are solving for and the 1 sandwich on the bottom because that is the
quantity that we started with. Sandwiches on the top and bottom cancel each other out,
and we are left with 7 times 2 slices of bread to make 14 slices of bread, which is our final
answer.
Notice that this is really a unit conversion problem, and the balanced equation is our
conversion factor.
Okay, now we are going to do the exact same thing with chemicals that we just did with
food. Check out this reaction. Nitrogen reacts with hydrogen to produce NH3, which is called
ammonia. This reaction is known as the Haber process, which is primarily used to make fertilizer.
Fertilizer helps crops grow and becomes part of our food. In fact, more than half of the
nitrogen in your body probably comes from this reaction, so...it’s pretty important.
As it’s written, the reaction is not balanced. There are two nitrogens on the left and only
one on the right, so we put a 2 in front of ammonia to balance the nitrogens. This gives
up 6 hydrogens on the right side but only two on the left, so we put a 3 in front of
hydrogen. Now, the reaction is all balanced. Remember, our balanced reaction is our recipe.
So, what is it actually telling us?
The balanced chemical equation tells us that one nitrogen molecule reacts with 3 hydrogen
molecules to make 2 ammonia molecules. Well, what if we have more? Let’s say 10 nitrogen
molecules? How many hydrogen molecules would they react with??? That’s right, 30, and
the reaction would produce 20 ammonia molecules. The balanced chemical equation tells us that
nitrogen, hydrogen, and ammonia are always in a 1 : 3 : 2 ratio no matter how big or
small the reaction is. So let’s talk about a large amount of nitrogen. The air you breathe
in every minute when you are walking contains approximately 600 billion trillion nitrogen
molecules, which is equal to 6 times 1023. Does that number look familiar? Yes, we’re
talking about 1 mole of nitrogen. And one mole of nitrogen will react with 3 moles of
hydrogen to make 2 moles of ammonia. Where am I getting those numbers? They are the big
numbers in front of the chemicals in the balanced reaction.
Let’s try a sample problem. How many moles of nitrogen do you need to make 10 moles of
ammonia. First, write down the quantity that you know. 10 moles of ammonia. Next, multiply
that by the ratio of nitrogen : ammonia in the balanced reaction. Since we are starting
the problem with ammonia, we put 2 NH3 on the bottom. This 2 NH3 comes directly from
the balanced reaction. Because we are solving for nitrogen, we put 1 N2 on the top. That
1 N2 also comes from the balanced reaction. Remember, if no number is written in front
of a chemical, that means the coefficient is one. Last of all, do the fraction multiplication,
10 times ½ is equal to 5. As for the units, notice that the ammonia we started with cancels
out with the ammonia on the bottom of the fraction, and we are left with moles of nitrogen,
which is exactly what we want.
Let’s try one more problem. How many moles of ammonia can you make from 0.37 moles of
hydrogen? First, write down the quantity that you know. 0.37 moles of hydrogen. Next, write
a fraction, and put 3 H2 on the bottom. The 3 H2 comes from the balanced reaction, and
the hydrogens cancel each other out. Then, because we are solving for ammonia, put 2
NH3 on the top. That 2 NH3 also comes from the balanced reaction. Finally, do the multiplication
in your calculator. If your calculator can’t do fractions or doesn’t have parentheses,
simply multiply 0.37 times 2, and divide that number by 3. The answer rounds to 0.25 moles
of ammonia.
And that’s it. Remember, to always use quantities in moles when doing Stoichiometry, and remember
that the balanced reaction is your conversion factor.
Thanks for watching. If this video helped you at all, give me a thumbs up. I really
do appreciate it. Feel free to also share any comments or questions you have below,
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