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Stoichiometry - Concept

Teacher/Instructor Kendal Orenstein
Kendal Orenstein

Rutger's University
M.Ed., Columbia Teachers College

Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.

Stoichiometry is the study of measuring or predicting the amount of reactants or products in a chemical reaction based on the variables such as the mass of reactants or products, the limiting reactant and the balanced chemical equation.

Alright so we're going to talk about Stoichiometry and Stoichiometry is the study of quantitative measurements between the amounts of reactants used and products formed in a chemical reaction and when we're talking about this, we're going to talk about the law of conservation of mass and don't forget the law of conservation of mass is that matter can neither be created nor destroyed meaning that we can't create matter out of nothing, we can't destroy matter in into nothing so understanding that we can look at balanced chemical equations and get lots of information from them.

Let's look at the first thing we can get. We can actually get the names of the compounds and relate them and their elements and saying that iron reacts with oxygen to form iron 3 oxide okay? Let me get the balanced chemical equation meaning we get the chemical symbols and the states of matter we say 4Fe solid plus 3 oxygen gas yield 2 Fe2O3 solid and what those coefficients actually mean that 4, 3 and 2 is that for every 4 atoms of iron that react with 3 molecules of O2 you get 4 formula sorry 2 formula units of Fe2O3.

Okay great and going a bit, step further, we can change these in the mols so there's actually molar relationship meaning that if we have 4 mols of iron reacting with 3 mols of oxygen gas, we can get 2 mols of iron 2 oxide oh sorry iron 3 oxide and if we'd get the molar masses of these it's in the periodic table we can say we have 223 grams of iron reacting with 96 grams of oxygen giving me 319.4 grams of iron 3 oxide and if you add these guys up on this side the mass of this it will equal the mass of the products, the mass of reactants will equal the mass of the products. Okay so understanding all these relationships we're going to take this one the one the molar relationship the one that we're going to actually going to use to actually expand on that and gets more information about the chemical reactions.

Okay so let's use this for example C3H8 plus oxygen gas yields carbon dioxide and water and it's a combustion reaction and its balanced for us and so we know that looking at the coefficients 1, 5, 3 and 2 we can say alright if I have one mol of butane which is C3H8, I need 5 mols of oxygen gas to fully react with that and then I'm going to get produce 3 mols of carbon dioxide and 2 mols of water if this will all in meaning if we started with 1 mol and 5 mols. But let us say we didn't start with 1 mol and 5 let's say instead of 5 mols we started with 3 mols because we're not always going to have this relationship let's say instead we started with started with 3 mols. How many mols of C3H8 I'm I going to need to react with that? Well I'm going to start at the beginning, I'm just going to do a molar relationship and say 3 mols which is the ratio mols of O2 and then I do what we call molar ratio so I'm going to cross this out so I look at my reaction I say for every 5 mols of O2 I require 1 mol of C3H8 to react with that so 3 divided by 5 is going to give me 0.6 so I actually need, C3H8, I if I had 3 mols of O2 to start with, I'm actually going to need only 0.6 mols of C3H8. Okay so how many, if I had 3 mols of O2 how many mols of water I'm going to be able to produce? So I'm going to do same thing, 3 mols unless I was given of 3 mols of O2 and I just going deal mol with a mol ratio so I have from my reaction I have 5 mols of O2 and I can produce 2 mols of H2O so 3 times 2 divided by 5 will give me 1.2 mol of H2O so I can use that molar relationship to really figure out everything on my chemical reaction to be able to compare all different molecules and elements that you have so in this to sum this up by 5, 3 mols of oxygen gas I need I'm sorry I can produce 1.2 mols of water. Okay great but I mean how often I going to use be able to use the word mols? Not very often you'll probably going to typically use mass and mass is in forms of grams so we actually have to a little bit further.

Let's take one step further let's say I have 7 mols of C3H8 here and I want to know how much water I'm going to produce what mass of water I'm I going to be able to produce. Okay, well the only way I can compare C3H8 and H2O is through the molar relationship its only the information I have so I'm going to start out with my what's given which is 7 mols of C3H8 and I'm going to do a molar relationship and do the mol ratio so if I have 1 mol in my my reaction I look and I have 1 mol of C3 oops H8 and I put it in the bottom side this can cross out and I require, it produces 2 mols of water okay but I'm not looking for mols I'm looking for grams I'm going to go step further I know that 1 mol of water is 18 grams from looking at my periodic table and I can cross these out because one's on top and one's on bottom and the question asked me for grams of water my I adding grams of water so I can stop here so I can multiply with whatever is on top and divide with whatever is on bottom so 7 times 2 times 18 divided by 1 divided by 1 is going to give me what, 252 grams of water so if I start out with 7 mols of C3H8 I actually produce 252 grams of water.

To actually go a little bit further because again we don't usually measure in mols I measure in grams we're going to do a mass to mass relationship and extend this just a bit further okay so instead of having I measured out 7 mols because we don't measure out in mols we measure in grams I'm given 27.2 grams of C3H8, how much oxygen gas I'm going to need to react with that fully? Well the only comparison I can have with these the only relationship I know is molar relationships so that I can change this to mols and then figure out the grams let's go from there. We start out with 27.2 grams of C3H8 okay so in a 1 mol of C3H8 oops it can get 44 grams of C3H8 and that I got from periodic table okay these cross out so now how many mols of C3H8 so great I know that 1 mol of C3H8 looking at my reaction I require 5 mols of oxygen gas and these gets to cross out because they're on top and bottom. My question asks for grams of oxygen gas right now I'm in mols of oxygen gas so I'm going to continue and say I know that 1 mol of oxygen gas is 32 grams looking at the periodic table so multiply whatever is on top and divide with everything that is in bottom I get 27.2 times 1 times 5 times 32 divided by 44 divide by 1 divided 1 which ends up giving me 98.9 grams of O2.

This reaction I mean not reaction this process and even though it's the longest one an I know it can get the most complicated its actually one you're going to be using much more often going from mass to mass and don't forget when you're going from mass to mass you have to go through mols because looking at our relationship this is the only piece of information that we have when comparing different things within the periodic table of the molar relationships we have to go through mols to get there so this is the basics of Stoichiometry.