Stoichiometry

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Transcript

Welcome to MCAT General Chemistry 1. We'll be covering stoichiometry, kinetics, equilibrium, and LeChatelier's Principle. Now let's start with some basics. First off, and empirical formula is often used for ionic and polymeric compounds in place of molecular formulas. Now these consist of simplest whole number ratio of elements in a compound.

So if we have hydrogen peroxide, we need divide out these 2s and we're left with an empirical formula of HO. For cyclohexane, C6H12, we're gonna divide 6 out and we're left with CH2, the empirical formula for cyclohexane. Now the molecular formula contains the actual number of atoms of each element in a compound, in their correct ratios.

It's not simplified down like an empirical formula. A mole is 6.02 x 10 23 somethings. So a mole of atoms would be 6.02 x 10 to the 23rd atoms. A mole of pencils would be 6.02 times 10 to the 23rd pencils. A mole is just a specific amount of something. Next up equivalents.

These are the number of moles of a particular atom in a mole of a compound and it's usually used to refer to the hydrogens in acids. For example, hydrogen chloride, hydrochloric acid has one equivalent of hydrogen, sulfuric acid has two equivalents of hydrogen, and phosphoric acid has three. The last two I ought to mention is that we can switch from mass to moles and vice versa using the molecular weight as a conversion factor.

So for example, carbon has a molecular weight of 12 grams per mole. If we had 24 grams of carbon, how many moles is that? Well, we can just take 24, multiply it by the inverse of the molecular weight. And find that we have 2 moles of carbon in 24 grams. What if we had half a mole of carbon? How many grams is that?

In this case, we multiply by the molecular weight, 12 grams per mole, and we get 6 grams of carbon in half a mole. Now stoichiometry essentially means element measuring and we use it to determine composition of compounds, or reaction products. First off we'll look at percent composition, two ways to do this. First we can do it by mass, percent composition by mass is equal to the grams of something over the grams total.

By mole, percent composition is equal to the moles of X over the moles total. So if we had some compound CH4 or methane, we could determine the percent composition of carbon, Would be the grams of carbon, over the grams of carbon plus the grams of hydrogen. The moles would be similar. The moles of carbon over the moles of carbon plus the moles of hydrogen.

So percent composition. Now in a reaction we might want to determine the percent yield. So if we take action to compound we get a new product. We want to know how much of that product we got versus what we would get theoretically if the reaction happened in completion. So percent yield is equal to an actual yield divided by a theoretical yield.

So for A plus B to C, if we expect to get 50 grams of C, but only get 20 grams instead, Our percent yield will be 20 over 50 which is a 40% yield. Now remember a percent yield, should be between 1 and 0. If you get a percent yield of 0, that means the reaction didn't happen. That percent yield of 1 means everything was completely converted.

If we get something greater than 1, that means we have some magic machine that we need to patent because we are making something from nothing. Now in reactions, we have what's called a limiting reagent, or the reactant that's consumed first, which then makes it so that further reaction is stopped. And the excess reagent, or the reactant that remains after the reaction is terminated.

So the limiting reagent runs out, leaving us with the leftover or the excess reagent.

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