 In this video I'm going to introduce you to a new term which describes the part of chemistry that allows us to make quantitative predictions about chemical reactions. The term is Stoichiometry. It was coined in the late 18th century by a German chemist called Jeremiah Richter, who about a decade before Dalton published his theory of atoms, did some significant work on how chemicals react in fixed proportions. He took the Greek word for element and the English suffix metri, meaning measurement, to give Stoichiometry and called it the art of chemical measurement. So Stoichiometry is the calculation of quantities in chemical reactions. For instance, if I take a hypothetical reaction like this, A reacting with B to give C, Stoichiometry lets us ask, I have 5 grams of A, how much B do I need to react with it, and how much C is going to be produced? Before we get into how this is done I have to remind you that all of this is based on the law of conservation of mass, that matter cannot be created or destroyed. In a chemical reaction we cannot create or destroy atoms, we can only rearrange them. So whatever atoms you start off with you must end up with exactly those atoms, no more and no less. This is the idea that lies behind the balancing of equations. Whatever number and type of atoms are on the reactant side, that's the exact number and type of atoms that must be on the product side. So here for instance you can see that we need to balance this equation because it looks like one of the two chlorine atoms on the left vanishes during the reaction. What that actually means is that it's impossible for one sodium atom and one chlorine molecule to give exactly one unit of sodium chloride. Instead by balancing the equation like this we find that what actually happens is that two sodium atoms react with one chlorine molecule to give exactly two units of sodium chloride. Now in its simplest form this equation means exactly what I just said, two sodium atoms react with one diatomic chlorine molecule to give two units of sodium chloride. But there's no reason that we can't scale it up. We could double it, four sodiums would react with two chlorine molecules to give four units of sodium chloride, or 100 atoms of sodium could react with 50 chlorine molecules to give 100 units of NACL, or to go to real life quantities two moles of sodium atoms could react with one mole of chlorine molecules to give two moles of sodium chloride. The big numbers in the balanced equations are called stoichiometric coefficients like the coefficients in algebra and they have the same function. They tell you how many you have of the thing that follows. More specifically the stoichiometric coefficients tell us in what ratio these chemicals react and are produced. But, and this is important, the ratio relates to numbers of atoms or molecules, or to moles of atoms or molecules if we go large scale. It does not relate to mass. Let me illustrate. Let's assume that we have two moles of sodium and one mole of chlorine gas and we react them together and they produce two moles of sodium chloride. How much would each of those quantities weigh? Well if we check the periodic table you'll see that sodium has a molar mass of 23 grams per mole so two moles of that would weigh 46 grams. Atomic chlorine has a molar mass of 35 grams per mole but chlorine is diatomic so the molar mass of chlorine gas is 70 grams per mole. And if we have one mole of that it'll weigh 70 grams. The product, the molar mass of sodium chloride is 58 grams per mole and we produce two moles of it so the mass produced is 116 grams. Alright firstly let's check that we haven't broken the law of conservation of mass. You can see that we have 116 grams of reactants in total which equals the mass of the product so that's okay. But now compare the masses to the stoichiometric coefficients. The chemical equation shows a one to one ratio between the sodium and the sodium chloride meaning for every two sodium atoms two units of sodium chloride are produced. But look at the masses. We used 46 grams of sodium and produced 116 grams of sodium chloride. This clearly doesn't obey the ratio and the reason is that different atoms weigh different amounts. Sure if you use two sodium atoms you'll be able to produce two units of sodium chloride but the sodium chloride is heavier. It has a whole extra chloride ion attached per unit. So remember the ratio of the stoichiometric coefficients represents the mole or molecule ratio not the mass ratio.