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stoichiometry

stoichiometry - Solomon's Study Notes General Chemistry I...

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S olomon’s Study Notes General Chemistry I Fall 2011 Solomon Weiskop PhD [ Stoichiometry ] General Chemistry 1

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Study Notes & Practice Problems are available to print out by registering at www.solomonlinetutor.com Solomon Weiskop PhD © Copyright 2011
1 1. Stoichiometry Stoichiometry is all about “amounts” Given a balanced chemical equation, Reactants → Products the typical stoichiometry problem is this: What amount of this corresponds to a certain amount of that ? Using stoichiometry you can relate amounts of anything to amounts of anything: You can relate an amount of a reactant to an amount of a product. You can relate an amount of a product to an amount of a reactant. You can relate an amount of a reactant to an amount of another reactant. You can relate an amount of product to an amount of another product. When doing stoichiometry calculations the amounts must be expressed as moles

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2 To illustrate, let’s consider the following simple balanced chemical equation: Reactants Products The stoichiometric coefficients in a balanced chemical equation are important. They tell us the relative amounts by which the reactants react to produce products. For example, in the chemical reaction above we can say that 1 mole of N 2 reacts with 3 moles of H 2 to produce 2 moles of NH 3 (*) (We could also say, if we wish, 1 molecule of N 2 reacts with 3 molecules of H 2 to produce 2 molecules of NH 3 ) However we cannot use the stoichiometric coefficients to tell us relative amounts in terms of mass (i.e. grams). That is, we cannot say 1 gram of N 2 reacts with 3 grams of H 2 to produce 2 grams of NH 3 Nonsense! This is nonsense. It suggests we can react 4 grams of reactants and produce only 2 grams of product. This is nonsense because it violates conservation of mass. Where did those other two grams go? Did they just disappear into thin air? Moral: Stoichiometry must be done using moles! There is only one exception to this. It will be discussed at the very end of these Study Notes. Another way to say (*) is this: In any chemical reaction, the reactants are reacting and the products are being produced. And all of this is happening according to the Molar Ratios provided by the stoichiometric coefficients in the balanced chemical equation.
3 For our simple reaction above, everything is happening in the ratio of 1-to- 3-to-2. Molar Ratio is one of the fundamental tools we will use for doing Stoichiometry problems. And again, for emphasis: Stoichiometry is done using moles . There is another fundamental tool we will need... To do Stoichiometry, we will use a Stoichiometer . What is a Stoichiometer, you ask? It is a wonderful machine, a marvel of modern technology, that has been specially designed to do stoichiometry. But stoichiometry must be done using moles.

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