Stoichiometry



Stoichiometry is all about ratios, chemical ratios. How many moles of this will react with how many moles of that. There are two levels of stoichiometry in chemistry. The first is composition stoichiometry. That is the one where you were looking at ratios of atoms that make molecules or formula units. The other one is reaction stoichiometry where are you are looking at how many moles of a compound will react with how many moles of another compound to make some sort of chemical product. In order to make stoichiometry work for you, you must have the correct chemical formulas for each substance and a correctly balanced chemical reaction.

Once you have your formulas and reactions in place, stoichiometry is really just a method of scaling things up or scaling things down. Let’s try a couple of examples of stoichiometry where things are scaled way up from the normal amounts we see in the formulas that we use.


Composition Stoichiometry Example

Iron(III)oxide has the formula Fe3O4 and is known in the mining world as magnetite. It is one of the the primary compounds in the ore that is mined so that we can produce iron, and more importantly - steel. So here is the question: how many kilograms of iron can be produced from 1.25 metric tons of iron ore in the form of magnetite? (note, on a question such as this, you should assume all the iron in the ore is extracted)

Answer: The first thing you should notice is the question is completely in mass units so we can answer this question by simply staying in mass units of iron oxide or magnetite. We first get its molar mass by taking the atomic weight of iron times three and the atomic weight of oxygen times four and adding the results. . The answer we get is 231.533 g/mol. Of that, 167.55 is iron. This can easily be turned into percent iron to get 72.36% iron. Now we use that percentage on the 1.25 metric tons of ore to get 0.905 metric tons of iron which is equal to 905 kg of iron. Here's what it looks like all written out.


Reaction Stoichiometry Example

Let's now consider a chemical reaction - a combustion. Propane reacts with oxygen to give carbon dioxide and water. How many pounds of carbon dioxide will be produced from 50 pounds of propane? How many kilograms is it?

STOP right there! You DO know how to write and balance a chemical reaction - right? That is standard high school chemistry stuff or Chemistry 101. I should probably write a section on it. OR, you could just do this on your own. Click that OpenStax link over there to the left. That is a really good start to learn this skill. Practice balancing chemical reactions. Do this first, THEN you do the stoichiometry calculations like what I show below.

Solution Step 1: Step 1 is to write out the correct formulas for the reactants and products. Pretty easy - just look them up on the internet if you don't already know them. Propane (C3H8), oxygen (O2), carbon dioxide (CO2), and water (H2O). Now put in the coefficients (those whole numbers in front of the formulas) on each side until the total number of atoms on each side of the arrow match. There are SO many places on the internet you can go to figure all this out and learn it. Just do it.

Solution Step 2: Step 2 is "doing the math" based on the balanced reaction that you made in step 1. Just to make it feel real, I've done this for this problem and my work is shown below. First, I wrote out the entire balanced equation with all the correct chemical formulas. Next, I use the molar masses of the reactants and products to calculate the answer. Note how I immediately just worked the problem in pounds first - that is, I used mass units of the propane and carbon dioxide. I converted to kg on the last step.



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