Percent Yield and Limiting Reagents

[Pages:15]Percent Yield and Limiting Reagents

February 17, 2011 Chapter 12

February 19, 2015

LIMITING REACTANTS

Limiting Reactants

? The substance that is used up first is the limiting reactant

? The substance that is left over after the reaction is the excess reactant

Something to Think About

? Given: 3 meat patties, 4 hamburger buns, 6 cheese slices, and 7 tomato slices

? Question: How many sandwiches can be made if each sandwich MUST have: 1 bun, 1 meat patty, 1 cheese slice, and 2 slices of tomato?

Cheeseburger Problem

? What ingredient(s) are left over (in excess)? How much of each is left?

? 1 bun, 3 cheese slices, 1 tomato

? What ingredient(s) are used completely (limiting)?

? Meat patties

Steps for Solving Limiting Reagent Problems

Limiting Reactant Problems involve 2 steps:

1. Identify the Limiting Reactant (LR)

? Calculate the number of moles obtained from each reactant in turn

? The reactant that gives the smaller amount of product is the Limiting Reactant

2. Calculate the amount of product obtained from the Limiting Reactant

? Set up a mole ratio to solve the problem

Example Problem #1

? A 1.4g sample of magnesium is treated with 8.1g of hydrochloric acid to produce magnesium chloride and hydrogen gas. How many grams of hydrogen are produced?

__Mg(s) + __ HCl(aq) __ MgCl2(aq) + __ H2(g)

Example Problem #1

? Balance the equation

Mg(s) + 2 HCl(aq) MgCl2(aq) + H2(g)

1.4 g 8.1 g

x g?

? Change masses of reactants in moles

Mg:

Limiting Reactant

1.4 g_ = 0.0576 Moles

24.3 g

HCl: Excess Reactant 8.1 g_ = 0.222 Moles 36.5 g

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