Acids and Bases. Arrhenius Acids and Bases

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1. What is the pH of the solution?

2. What percent of the parahydroxybenzoic acid has ionized in this solution?

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D. Comment on Polyprotic Acids

Example: H3PO4

Ka’s = 7.5 x 10-3, 6.2 x 10-8, 3.6 x 10-13

1. Show the three acid ionization equations with their accompanying Ka values.

2. Which is the strongest acid?

3. How would you go about calculating the pH of a given H3PO4 solution?

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Weak Base Equilibrium Rxns

Base Ionization Constant

(base will accept proton from H2O, forming OH-)

1. Definition

Give the base ionization constant expression for the base: B(aq) + H2O(l) ⇌ BH+(aq) + OH-(aq)

Kb = ?

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2. Relative Strength of Weak Bases

a. The Kb for ammonia is 1.8 x 10-5.

The Kb for phosphate ion is 2.8 x 10-2.

Which is the strongest base?

b. Which will have a higher pH:

a 1.0 M ammonia solution, or

a 1.0 M phosphate ion solution

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B. Kb From Equilibrium Concentrations

The pain killer, morphine, is a weak base. A 0.01 M morphine solution has a pH of 10.1. Calculate the Kb for morphine.

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C. pH Calculations from Kb of Weak Base

The weak base methylamine (CH3NH2) has a Kb of 5.0 x 10-4. Answer the following questions for a 0.080 M aqueous solution of methylamine.

a. Write the chemical equation.

b. Write the base ionization equilibrium expression.

c. Calculate the [OH-], pOH, and pH for solution.

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Relationship of Ka and Kb

A. Conjugate Acid and Base Reaction May Be Written in Both Directions. For Example:

HA + H2O ⇌ H3O+ + A- Ka =

A- + H2O ⇌ HA + OH- Kb =

** Reaction will go in direction of stronger acid and base to the weaker acid and base.

** Compare Ka and Kb to decide direction.

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B. Mathematical Relationship of Ka and Kb

(for previous equations)

Thus:

Ka x Kb = [H+] [OH-] = Kw

Ka x Kb = 1 x 10-14

and pKa + pKb = 14

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C. Example Problems

Consider the dihydrogen phosphate ion, H2PO41-. Ka = 6.2 x 10-8.

1. What is its conjugate base?