Stoichiometry - Online Tutor, Practice Problems & Exam Prep

Stoichiometry

When it comes to chemical equations, it becomes important to first balance your chemical equation because this will later on lead us to our understanding of stoichiometry. Now stoichiometry deals with the numerical relationship between compounds in a balanced chemical equation. It allows us to determine the amount of products from reactants and vice versa. For example, here we have our chemical equation that's balanced. We have 2 hydrogen gases reacting with 1 oxygen gas to produce 2 water molecules as gas. They're giving us 12.3 grams of H₂ and they're asking us to determine the grams of H₂O, H₂O gas produced. This is what stoichiometry is. They're giving us a balanced chemical equation, they're providing us information on one of the compounds within this balanced equation, and asking us to find another compound within that same balanced equation. Now that we know what stoichiometry is, how exactly do we do it? Well, if you click on the next video, we'll take a look at the procedures that you'll need to employ in order to do any type of stoichiometric calculation.

Stoichiometry

So as we said, we're given 12.3 grams of H₂ and asked to find how many grams of H₂O would form. The way we're able to do this is through stoichiometry. And to find how many grams, we'd use the stoichiometric chart. Now the chart uses the given quantity of a compound to determine the unknown quantity of another compound. The way it works is we're starting here on the left side and on the left side, the information they give to us is called the given information. Now, this information that's given to us can be presented as grams, and if it's presented as grams, then we can just do a conversion to get to moles. So, we can go from grams of given to moles of given. Now, besides giving it to us in grams, it could be given to us in ions, atoms, formula units, or molecules of given. Again, we're heading towards moles of given. So our basic movement is going from the left side of the chart toward the right side of the chart to our unknown information.

Now that we have our moles of given, we have to basically take a jump, a leap of faith from an area where we know information because it's given to us, to an area where we know nothing at all, so it's unknown to us. And this jump or leap of faith that we're going to take, we call it the jump. To be able to do this jump, you have to do a mole to mole comparison. And in a mole to mole comparison, use the coefficients in the balanced equation. That's why it's incredibly important you first have a balanced equation before you even attempt to do stoichiometry. Once we've gone from moles of given to moles of unknown, and then we can either stop there if they want us to find the answer in moles, or continue onward to ions, atoms, formula units, or molecules of unknown or to grams of unknown. In this case, from the example that we have up above of our equation, we would say that this is the grams given to us, so this would be our grams of given.

We would have to basically take the trip of grams of given to moles of given to moles of our unknown, our unknown is what we're looking for which would be water, and then we'd have to stop here at grams of unknown. That's the path we have to take. We utilize the stoichiometric chart to help us answer any type of stoichiometric question. Now that we've seen this chart, move on to our example question and let's put it to work, put it to practice in terms of this stoichiometric chart.

Stoichiometry Example 1

Here in this example question it says, how many grams of H₂O are produced when 12.3 grams of H₂ reacts? Alright. So we know that they're giving us information on one compound in a balanced equation and asking for information on another. We know that this is the definition of stoichiometry. So we're going to have to utilize the stoichiometric chart in some way to solve this problem. Now, if we follow the steps, it says step 1, map out the portion of the stoichiometric chart you will use. From the question, they're giving us 12.3 grams of H₂. Since that's a value they're giving to you, that represents our grams of given. So we're gonna start at grams of given, which is just grams of H₂, and we're gonna convert those grams into moles of given, so moles of H₂. Now in the same question they're asking us to find the grams of H₂O. Since we don't know they're asking us to find it, this represents our grams of unknown. So that tells me I have to go for moles of given and find a way to get to grams of unknown. Now at this point, to go for moles of given, I have to go to moles of unknown and that is where it's required to do the job. So we're gonna go from moles of given to moles of unknown. And then finally, we go from moles of unknown to grams of unknown. This is the path that we're going to take to answer this question. So let's go to step 2. It says convert the given quantity into moles of given. And if a compound is set to be in excess, then just ignore it. In this question, they don't say anything as being in excess, so we don't have to worry about this, next line under step 2. Later on, we'll come into situations where we're told something is an excess. Excess, and in that case, we just simply ignore it. Alright. So we're going to take our given quantity, which is 12.3 grams H₂, and we're going to convert it into moles of given. Grams of H₂ go on the bottom, and 1 mole of H₂ goes on top. H₂ has in it 2 hydrogens, and according to the periodic table, each one weighs 1.008 grams, so that's 2.016 grams. Grams here cancel out and I'll have moles of H₂ which comes out to 6.1012 moles of H₂. So I've just gone from grams of H₂ to moles of H₂. At this point, we have to do the jump. So going to step 3, it says to do a mole to mole conversion in order to..."

convert moles of given into moles of unknown. So we take that 6.1012 moles of H₂. To get rid of moles of H₂, I put them on the bottom. What am I looking for? I'm looking for my unknown. My unknown is water, so I need to find moles of H₂O. Now remember, to go from moles of given to moles of unknown, that's called the jump, and we do a mole to mole comparison. The equation says that for every 2 moles of H₂, I have 2 moles of H₂O. So for every 2 moles of H₂, I have 2 moles of H₂O. So here, moles of H₂ cancel out, and now I have moles of H₂O. So that's 6.1012 moles of H₂O. Now finally, if necessary, convert the moles of unknown into the final desired units. Sometimes they may ask us to just find the moles of our unknown, and in that case, we'd stop. For this particular question though, they're not asking us to find moles of water, they're asking us to find grams of water. So an additional step is required. So 6.1012 moles of H₂O. Gotta get rid of moles of H₂O, so one mole of H₂O on the bottom. How many grams of H₂O do we have on top? H₂O is composed of 2 hydrogens and one oxygen. Hydrogen, according to the periodic table, is 1.008 grams. Oxygen is 16 grams. This is 2.016 grams, and this is 16 grams, so the complete mass of H₂O is 18.016 grams. So we plug that here. Moles of water cancel out, and now I'm finally gonna have grams of water. So here we're going to multiply those out, so that's 109.9 grams of H₂O. Now, technically, within this question, 12.3 has 3 significant figures. So technically we should write this in 3 significant figures as well. So in scientific notation that will come out to 1.10 times 10 to the 2 grams of H₂O. So this would be our final answer for this stoichiometric question. So remember, if we are given the amount of a compound within a balanced equation and asked to find another, we're dealing with stoichiometry, which means you have to utilize the stoichiometric chart in order to find your final answer.

The oxidation of chromium solid is represented by the following equation:

4 Cr (s) + 3 O₂ (g) → 2 Cr₂O₃ (s)

How many moles of chromium (III) oxide are produced when 34.69 g Cr reacts with excess oxygen gas? 

• A. 0.334 mol
• B. 1.33 mol
• C. 0.749 mol
• D. 3.00 mol
• E. 0.667 mol

The reaction of potassium chlorate and sucrose is given below:

8 KClO₃ + C₁₂H₂₂O₁₁ → 8 KCl + 12 CO₂ + 11 H₂O

If 2.33 x 10^-7 formula units of potassium chlorate are reacted, how many grams of carbon dioxide will be produced? 

• A. 3.94x10^-16 g CO₂
• B. 8.23x10^-23 g CO₂
• C. 2.72x10^-21 g CO₂
• D. 7.81x10^-23 g CO₂
• E. 2.55x10^-29 g CO₂

If the density of ethanol, CH₃CH₂OH, is 0.789 g/mL, how many milliliters of ethanol are needed to produce 4.8 g of H₂O in the following reaction?

CH₃CH₂OH (l) + 3 O₂ (g) → 3 H₂O (l) + 2 CO₂ (g)

• A. 5.2 mL
• B. 47 mL
• C. 3.2 mL
• D. 29 mL
• E. 74 mL