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Unformatted text preview: 3.11 The Concept of Limiting Reagent 113 What is the mole ratio between N2 and CuO in the balanced equation? 1 mol N2 3 mol CuO What are the moles of N2? 1 mol N2 1.14 Jll.Gl-€uO X 3 Jll.Gl-€uO
= 0.380 mol N 2 What mass of N2 is produced? Using the molar mass of N2 (28.02 g/mol), we can calculate the mass of N2 produced: • 0.380 mel:-N2 X 28.02 g N2 1 mel:-N2
= 10.6 g N2 SEE EXERCISES 3.109 THROUGH 3.111 The amount of a product formed when the limiting reactant is completely consumed is called the theoretical yield of that product. In Example 3.17, 10.6 grams of nitrogen represents the theoretical yield. This is the maximum amount of nitrogen that can be produced from the quantities of reactants used. Actually, the amount of product predicted by the theoretical yield is seldom obtained because of side reactions (other reactions that involve one or more of the reactants or products) and other complications. The actual yield of product is often given as a percentage of the theoretical yield. This is called the percent yield: - __ :ent yield is important as an indicator ~ - efficiency of a particular laboratory ustrial reaction.
----=--- Actual yield X Theoretical yield 100% = percent yield For example, if the reaction considered in Example 3.17 actually gave 6.63 grams of nitrogen instead of the predicted 10.6 grams, the percent yield of nitrogen would be 6.63 g-N2
--X 10.6 g-N2 100% = 62.5% EXAMPLE 3.18 Methanol (CH30H), also called methyl alcohol, is the simplest alcohol. It is used as a fuel in race cars and is a potential replacement for gasoline. Methanol can be manufactured by combining gaseous carbon monoxide and hydrogen. Suppose 68.5 kg CO(g) is reacted with 8.60 kg H2(g). Calculate the theoretical yield of methanol. If 3.57 X 104 g CH30H is actually produced, what is the percent yield of methanol?
Where are we going? • To calculate the theoretical yield of methanol • To calculate the percent yield of methanol What do we know? ./ The chemical reaction ...
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