# Acids, pH and EquilibriumPage 1

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Søren Sørensen

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## Acid/Base Definitions

Arrhenius Model

Acids produce hydrogen ions in aqueous solutions

Bases produce hydroxide ions in aqueous solutions

Bronsted-Lowry Model

Acids are proton donors

Bases are proton acceptors

Lewis Acid Model

Acids are electron pair acceptors

Bases are electron pair donors

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## Acid Dissociation

HA  H+ + A-

Acid Proton Conjugate

base

Alternately, H+ may be written in its hydrated form, H3O+ (hydronium ion)

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## Dissociation of Strong Acids

Strong acids are assumed to dissociate completely in solution.

Large Ka or small Ka?

Reactant favored or product favored?

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## Dissociation of Weak Acids

Weak acids are assumed to dissociate only slightly (less than 5%) in solution.

Large Ka or small Ka?

Reactant favored or product favored?

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## Self-Ionization of Water

H2O + H2O  H3O+ + OH-

At 25, [H3O+] = [OH-] = 1 x 10-7

Kw is a constant at 25 C:

Kw = [H3O+][OH-]

Kw = (1 x 10-7)(1 x 10-7) = 1 x 10-14

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Calculating pH, pOH

pH = -log10(H3O+)

pOH = -log10(OH-)

Relationship between pH and pOH

pH + pOH = 14

Finding [H3O+], [OH-] from pH, pOH

[H3O+] = 10-pH

[OH-] = 10-pOH

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pH and pOH Calculations

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The pH Scale

Graphic: Wikimedia Commons user Slower

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## A Weak Acid Equilibrium Problem

What is the pH of a 0.50 M solution of acetic acid, HC2H3O2, Ka = 1.8 x 10-5 ?

Step #1: Write the dissociation equation

HC2H3O2  C2H3O2- + H+

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