Calculating the pH of weak acid solutions

Here is a generic statement of the problem to solve:

Determine the pH of a _____ M solution of _____ acid, Ka = _____.

Notice that the molarity, acid name and Ka are not there. That is because the solution technique is the same for almost all weak acids.

The following list gives sets of names and values that can be inserted into the above problem. The boldface first letter of the acid name and Number will be used in the assignment given below the table.

Name of Acid Ka Number Molarity
Anisic 3.38 x 10¯5 1 0.150
Cyanoacetic 3.65 x 10¯3 2 0.200
Ethylbenzoic 4.47 x 10¯5 3 0.325
Hexanoic 1.31 x 10¯5 4 0.475
Methylmalonic 1.17 x 10¯4 5 0.550
Trimethylacetic 9.40 x 10¯6 6 0.700

The Acid Assignment

1) Choose an acid from the above list. Please use the Ka listed just to the right of the acid name.

2) Now pick a molarity. It DOES NOT have to be in the same row as your acid.

3) Rewrite the generic statement of the problem, filling in the blanks with the specific acid name, Ka and molarity.

4) Solve the problem in this manner:

a) write the dissociation equation for the acid dissolving in water solution. Use the boldfaced letter to represent the anion (the part remaining after the acid donates its proton).
b) write the Ka expression for the specific acid.
c) calculate the pH of the acid solution, showing sufficient steps to justify your answer.

5) Pick a different acid and a different molarity and follow steps 3 and 4. Do this twice for a total of three problems completed.


Calculating the pH of Weak Base Solutions

Here is a generic statement of the problem to solve:

Determine the pH of a _____ M solution of _____ , Kb = _____.

Notice that the molarity, base name and Kb are not there. That is because the solution technique is the same for almost all weak bases.

The following list gives sets of names and values that can be inserted into the above problem. The boldface first letter of the base name and Number will be used in the assignment given below the table.

Name of Base Kb Number Molarity
Aniline 4.62 x 10¯10 1 0.150
Codeine 1.62 x 10¯6 2 0.200
Ephedrine 9.08 x 10¯5 3 0.325
Nicotine 1.05 x 10¯6 4 0.475
Pyridine 1.78 x 10¯9 5 0.550
Strychnine 1.82 x 10¯6 6 0.700

The Base Assignment

1) Choose a base from the above list. Please use the Kb listed just to the right of the base name.

2) Now pick a molarity. It DOES NOT have to be in the same row as your base.

3) Rewrite the generic statement of the problem, filling in the blanks with the specific base name, Kb and molarity.

4) Solve the problem in this manner:

a) write the ionization equation for the base dissolving in water solution. Use the boldfaced letter to represent the base (which accepts a proton).
b) write the Kb expression for the specific base.
c) calculate the pH of the base solution, showing sufficient steps to justify your answer. Remember the additional step you must use at the very end of the problem.

5) Pick a different base and a different molarity and follow steps 3 and 4. Do this twice for a total of three problems completed.