Acids and Bases: Salts

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Introduction

Molecular Structure

Ionization Constants

Salts

Buffers

Lewis Theory

Salts of Weak Acids and Weak Bases

When formic acid reacts with ammonia, ammonium formate is produced:

HCO2H + NH3 NH4HCO2

You have already determined that NH4+ is the weak conjugate acid of NH3 and HCO2- is the weak conjugate base of HCO2H.

If the cation of the salt causes the pH to decrease and the anion of the salt causes the pH to increase, how can the pH of the solution be determined? The answer lies in the relative strengths of the acidic cation and the basic anion.

As previously determined, ammonium is a weak acid and will produce hydronium ions in water while formate is a weak base and will produce hydroxide ions in water:

NH4+ + H2O NH3 + H3O+

HCO2- + H2O HCO2H + OH-

The Kb for ammonia (NH3) is 1.8 x 10-5; the Ka for formic acid is 1.8 x 10-4. Calculate the ionization constants for their conjugate acid and base and enter them below.

NH4+ + H2O NH3 + H3O+

 Ka =

HCO2- + H2O HCO2H + OH-

 Kb =

Given the Kb of a weak base, the Ka of its conjugate acid can be calculated:

Kw = Ka x Kb

Given the Ka of a weak acid, the Kb of its conjugate base can be calculated:

Kw = Ka x Kb

Of the two reactions shown above, click on the more product-favored reaction.

Remember the larger the ionization constant, the more product-favored the reaction.

Good!

Will a NH4HCO2 solution be acidic, basic, or neutral?

Acidic Basic Neutral

Good! Since the acid ionization of NH4+ is more favored than the base ionization of HCO2-, the solution will be acidic.

Since the acid ionization reaction is more favored than the base ionization reaction, there will be more H3O+ than OH- in solution.

Summary

Use the figure below to review and test yourself on how various salts affect the pH of an aqueous solution.

Click on strong or weak acid and strong or weak base and see how various combinations affect the pH of a solution.

Salts