Chapter 4: Chemical Quantities

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Chapter 4 Chemical Quantities Aqueous Reactions

• CHE 123 General Chemistry I
• Dr. Jerome Williams, Ph.D.
• Saint Leo University

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Overview

• Limiting Reactants
• Solutions An Introduction
• Solution Stoichiometry

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Limiting Reactant

• Limiting Reactant (Reagent) is defined as the
  reactant that is consumed first in a chemical
  reaction and, therefore, limits the amount of
  products that form.

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Limiting Reactant

• A Practical Example
• Recipe
• 1 crust 5 oz. sauce 2 cups cheese gt 1 pizza

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Limiting Reactants

• Scenario 1 Reactants are in 11 Ratio
• Reactant with smallest no. moles is limiting.
• A B ? C
• 0.5 moles 0.25 moles ? moles
• The answer has to be 0.25 moles because when
  reactant B is used up, the reaction STOPS!

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Limiting Reactants

• Scenario 2 Reactants are NOT in 11 Ratio
• Must divide mole quantities by their coefficients
  in balanced reaction to establish which reactant
  is limiting.
• A 3 B ? 2 C
• 2.50 moles 4.50 moles ? moles
• The limiting reactant is B despite the higher no.
  moles when compared to reactant A.

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Limiting Reactants

• Scenario 2 Reactants are NOT in 11 Ratio
• A 3 B ? 2 C
• 2.50 moles 4.50 moles ? moles
• 2.50 moles A (4.50 moles B / 3) 1.50 mol B
• B is limiting reactant because it reacts three
  times for every one time A reacts, despite having
  the higher no. moles when compared to reactant A.

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Limiting Reactants

• Scenario 2 Reactants are NOT in 11 Ratio
• A 3 B ? 2 C
• 2.50 moles 4.50 moles ? moles
• How many moles product will form?
• The answer is 3.00 moles. Verify that this is
  true.

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Example Problem

• If we react 42.5 g Mg and 33.8 g O2, what is
  limiting reactant theoretical yield?
• Reaction 2 Mg O2 gt 2 MgO

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Example 4.3

• Starting with 86.3 g NO and 25.6 g H2, find the
  theoretical yield of NH3 in grams.
• Rxn 2 NO (g) 5 H2 (g) ? 2 NH3 (g) 2 H2O (g)

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Solutions An Introduction

• A solution is a homogeneous mixture that consists
  of two components.
• Solute is substance being dissolved in the
  solution.
• Solvent is the dissolving environment.
• Most chemical reactions are carried out in liquid
  state or in solution.
• In biology general chemistry, your solvent is
  water unless told otherwise.

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Solutions An Introduction

• Molarity is an expression of solution
  concentration. It is defined as the number of
  moles of solute divided by the volume of a
  solution in liters.
• Molarity M moles solute / Vol. soln. in
  Liters
• By simple rearrangement one obtains the following
  equations.
• (Vol. soln. in Liters ) x (M) moles solute
• Vol. soln. in Liters moles solute / M

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Solutions An Introduction

• A dilution is a process by which water (or
  another solvent) is added to a solution of known
  concentration to achieve a new solution of lower
  concentration.
• Dilution does not alter number of moles of
  solute.
• Dilution equation
• M1V1 M2V2

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Solutions An Introduction

• Some example problems to consider.
• What is the molarity of a solution prepared by
  adding 24.66 g of potassium nitrate to 250 mL of
  water?
• Answer 0.975 M
• How does one prepare 250 mL of 1.50 M HCl from
  12.0 M HCl ?
• Answer Measure 31.25 mL of 12.0 M HCl and add
  to 218.75 mL of distilled water. Mix contents
  well before using.

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Solution Stoichiometry

• Handy Dandy Five Step Method is still used.
• The only modification is now we have volumes and
  concentrations to consider.
• Use Molarity equations to find number of moles
  and then proceed as before.

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Solution Stoichiometry
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Solution Stoichiometry

• An example involving solutions.
• Sodium hydrogen carbonate (NaHCO3) is used as an
  antacid. One tablet requires 34.50 mL of 0.138M
  HCl solution for complete reaction. Determine the
  number of grams of NaHCO3 that one tablet
  contains
• The balanced chemical reaction is shown below.
• NaHCO3 HCl ? NaCl H2O CO2
• Answer 0.400 grams NaHCO3