In the reaction, their mole ratio is 1:2, meaning that we need twice as many moles of oxygen as we do methane. Let’s get to it! First of all, let’s look at the expected product. This concept is very important to the applications of stoichiometry. The core concept we can take away from it is that one molecule of methane combines with two molecules of diatomic oxygen to form one molecule of carbon dioxide and two molecules of water, or correspondingly, that one mole of methane combines with two moles of diatomic oxygen to form one mole of carbon dioxide and two moles of water. This balanced equation tells us a lot about how this reaction works. You’ve probably learned much of this even in a non-AP® Chemistry class, so bear with me it’ll get more advanced. So, we double the number of O_2 on the left side, yielding: In fact, it’s even worse than before the ratio is now 2:4. However, now we still have more oxygen on the right side. The only way to get that four hydrogens is to double the amount of H_2O on the right side. Uh-oh! How do we balance this equation? First, we look at the left side and see that we need four hydrogens. Then I do the same on the right side: one carbon, two hydrogens, and three oxygens. How can I tell? I add up the number of atoms of each element on the left side: one carbon, four hydrogens, and two oxygens. Let’s say that we have this equation, which is the combustion of methane. First of all, we have to start with a balanced reaction. In this section of the AP® Chemistry Crash Course, we’ll start by looking at the basic concepts of stoichiometry, and then we’ll cover five applications for stoichiometry on the AP® Chemistry exam.įirst, let’s talk about the basic concepts of stoichiometry. When we look at the two sides of the reaction, we have to make sure that the number of atoms of each element on each side is the same because of the principle of conservation of mass. Of course, that means that we need to start with a chemical reaction. We’ll go into more detail about that later. In other words, stoichiometry is the practice of using a chemical reaction equation to predict the results of the reaction. Stoichiometry derives from the Greek stoicheionmetron, meaning “element measure”. Stoichiometry is one of the most important topics on the AP® Chemistry exam, so it’s vital that you understand it and all of its applications.
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