Popular in Chemistry 142
Popular in Chemistry
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Date Created: 11/02/14
CH 3 NOTES In this chapter we will consider the quantities of materials consumed and produced in chemical reactions This are of study is called chemical stoichiometry To understand chemical stoichiometry you must first understand the concept of relative atomic masses 31 Atomic Masses The modern system of atomic masses instituted in 1961 is based on Carbon12 as the standard In this system Carbon12 is assigned a mass of exactly 12 atomic mass units amu The most accurate method currently available for comparing the masses of atoms involves the use of the mass spectrometer Because natural carbon is a mixture of isotopes the atomic mass we use for carbon is an average value based on its isotopic composition 1201 not 12 NO atom of carbon probably has exactly mass 1201 In addition to being used for determining accurate mass values for individual atoms the mass spectrometer is used to determine the isotopic composition of a natural element 32 The Mole Because samples of matter typically contain so many atoms a unit of measurement called the mole has been established for use in counting atoms For our purpose it is most convenient to define the mle as the number equal to the number or carbon atoms in exactly 12 grams of pure Carbon12 Modern techniques that allow us to count atoms very precisely have been used to determine this number as 6022E23 This number is called Avogadro s number to honor his contribution to chemistry One mole of something consists of 6022E23 units of that substance Thus the mole is defined such that a sample of a natural element with a mass equal to the element39s atomic mass expressed in grams contains 1 mole of atoms Paying careful attention to units and making sure the answer is sensible can help you detect an inverted conversion factor or a number that was incorrectly entered in your calculator 33 Molar Mass The terms molar mass and molecular weight mean exactly the same thing the mass in grams of 1 mole of a compound Some substances exist as a collection of ions rather than as separate molecules In this text for convenience we will apply the term molar mass to both ionic and molecular substances In some texts the term formula weight is used for ionic compounds instead of the terms molar mass and molecular weight 34 Conceptual Problem Solving One of the great rewards of studying chemistry is to become a good problem solver It is our purpose in this text to help you learn to solve problems in a flexible creative way based on understanding the fundamental ideas of chemistry We call this approach conceptual problem solving A main goal in conceptual problem solving is to get the big picture a real understanding of the situation This approach to problem solving looks within the problem for a solution The following organizing principles will be useful to us as we proceed to solve a problem 0 First we need to read the problem and decide on the final goal 0 We need to work backward from the final goal to decide where to start 0 After getting an answer we should check to see whether the answer is reasonable 35 Percent Composition of Compounds The mass percent often called the weight percent of carbon in ethanol can be computed by comparing the mass of carbon in 1 mole of ethanol with the total mass of 1 mole of ethanol and multiplying the result by 100 No matter how complicated the problems become you should always know specifically what you are trying to solve and then ask and answer questions to get the solution Your knowledge is important as well and in chemistry this knowledge builds 36 Determining the Formula of a Compound When a new compound is prepared one of the first items of interest is its formula The formula is often determined by taking a weighed sample of the compound and either decomposing it into its component elements or reacting it with oxygen to produce substances such as CO2 H20 and N2 which are then collected and weighed We can find the smallest whoe number ratio of atoms in a compound by dividing each of the mole values above by the smallest of the three mole values The empirical formula represents the simplest whoe number ratio of the various types of atoms in a compound TO be able to specific the exact formula of the molecule involved the molecular formula we must know the molar mass 37 Chemical Equations A chemical change involves reorganization of the atoms in one or more substances This process is represented by a chemical equation with the reactants on the left side of the arrow and the products on the right side Notice that the atoms have been reorganized Remember that in a chemical reaction atoms are neither created nor destroyed All atoms present in the reactants must be accounted for among the products 0 In other words there must be the same number of each type of atom on the product side as there is on the reactant side of the arrow Making sure that this rule is followed is called balancing a chemical equation for a reaction 0 The chemical equation for a reaction provides two important types of information the nature of the reactants and products and the relative number of each The reactants and products in a specific reaction must be identified by experiment 38 Balancing Chemical Equations 0 Whenever you see an equation you should always check to see it39s balanced When the equation for a reaction is balanced the identities of the reactants and products must not be changed The formulas of the compound must never be changed when balancing an equation Most chemical equations can be balanced by inspectionthat is by trial and error It is always best to start with the most complicated molecules those containing the greatest number of atoms 39 Stoichiometric Calculations Amounts of Reactants and Products Calculation of Masses of Reactants and Products in Chemical Reactions 0 Balance the equation for the reaction 0 Convert the known masses of the substances to moles 0 Use the balanced equation to set up the appropriate mole ratios 0 Use the appropriate mole ratios to calculate the number of moles of the desired reactant or product 0 Convert from moles back to grams if required by the problem 310 Calculations Involving a Limiting Reactant When chemicals are mixed together to undergo a reaction they are often mixed in stoichiometric quantities that is in exactly the correct amounts so that all reactants run out are used up at the same time In any stoichiometry problem it is essential to determine the limiting reactant or limiting reagent to calculate correctly the amounts of products that will be formed 0 The amount of a given product formed when the limiting reactant is completely consumed is called the theoretical yield of that product The actual yield of product is often given as a percentage of the theoretical yield This value is called the percent yield 0 Percent yield actual yieldtheoretical yield x 100 Practice Problems given by Professor 367 377 310