) CChieh@UWaterloo.ca

## Qz6 - Acids, Bases and Salts

### Skills to be tested

• Calculate the pH, concentrations of various equilibrium species of acid and base solutions.
• Apply the concept of conjugate acid-base pairs to solve problems.
• Consider autoionization of water in problem solving.
• Evaluate the fraction of dissociation.
• Calculate the pH at various stages of titration.
• Apply equilibria of polyprotic acids in problem solving.
• Apply the hydrolysis principle to evaluate pH of salt solutions.
• Evaluate quantities in buffer solutions.
• ## Review of Acids, Bases and Salts

This Quiz covers the equilibria of acids, bases, and salts. When you dissolve a salt in water to make a solution, the cations or anions or both will react with water (hydrolysis), resulting in an acidic or basic solution. The quantities to be calculated are pH, and the concentrations of the various species.

Some of the quiz questions have a difficulty level expected in term tests. You might get questions of similar difficulty in the final exam.

The common ion effect is a broader view of buffer solutions. In doing this quiz, it is important to analyze the problem carefully, determine what information you have available, and then what is required of you to do.

If you are using the DOS version, you may press the (Q) key to enter the Quiz mode.

### Review Problems

Here are some example problems that involve neutralization, buffer, hydration, salts and titrations.

• What is the pH for a 1.0 M HIO3 solution if its pKa=0.80?

Hint

```HIO3 = H+ + IO3-
1.0-x   x    x           (Equilibrium concentration)

x2
----- = 10-0.8 = 0.18.
1 - x
```
Find the solution for this quadratic equation, x = ? and then evaluate pH = ?

• When 1.0 mol HIO3 is added to pure water to give 1.0 L of solution, the pH is 0.484. Calculate Ka and pKa for HIO3.

Hint

Well, this is a reverse of the previous question. Set up the equations and solve it to see if you get the right values for Ka and pKa.

### Confidence Building Questions

• A solution contains 0.20 M NaH2PO4 and 0.10 M Na2HPO4.
The ionization constants for H3PO4 are:
Ka1 = 7.52e-3, Ka2 = 6.23e-8, Ka3 = 4.5e-13.
Calculate the pH.

Hint...
H2PO4- is an amphiprotic ion. Its ionization should be considered. HPO42- is the common ion between the two salts.

Consider...
Is this a buffer solution?

• A solution contains 0.10 M HCl and 0.10 M acetic acid, CH3COOH, Ka = 1.8e-5. What is the concentration of CH3COOH?

Hint...
Since [H+] = 0.10, the percentage of ionization of acetic acid is very low. Nearly all molecules are CH3COOH.

Consider...
[CH3COOH] = 0.10 M. [CH3COO-] = ?

• A solution contains equal concentrations of NH3 and NH4Cl.
Kb for NH3 is 1.77e-5. Calculate its pH.

Hint...
Consider either common ion effects or buffer solutions.

Consider...
This is a buffer solution.

• What is the pH of a solution containing 1.0 M HIO3 and 1.0 M acetic acid, pKa = 4.75?

Hint
Since the Ka value for acetic acid is so small, its presence does not change the pH from a solution containing only 1.0 M HIO3. DO NOT take these words for it, set up two equilibrium equations and calculate the results to see the small effect of accetic acid.

After you have done the calculation, you may want to make a conclusion: The pH is determined by the stronger acid if the two acids have very different pKa or Ka values.

• What is the pH of a solution containing 1.0 M NaIO3?

Hint
Consider the hydration of the weak base IO3- in the solution. The calculation is easy. Note how this same problem is worded in the next problem.

• What is the pH of a mixture of 10.0 mL 1.0 M HIO3 and 10.0 mL 1.0 M NaOH?

Hint
Well, the end result is a 0.50 M NaIO3 (salt) solution. This is a hydration problem.

• What is the pH of a mixture of 10.0 mL 1.0 M HIO3 and 10.0 mL 1.0 M NaIO3?

Hint
This is a buffer problem. Under the circumstance, the pH is the pKa of the weak acid.

• What is the pH of a mixture of 10.0 mL 1.0 M NaIO3 and 10.0 mL 1.0 M NaCl?

Hint
The NaCl solution is neutral. It does not affect the pH of the pure NaIO3 solution.

• What is the pH of a mixture of 10.0 mL 1.0 M NaIO3 and 10.0 mL 1.0 M HCl?

Hint
The result solution is 0.50 M in NaCl, and 0.50 M HIO3. You have done the calculations for this problem earlier.

• What is the pH of a mixture of 10.0 mL 1.0 M NaIO3 and 5.0 mL 1.0 M HCl?

Hint
This is again a buffer solution.
[HIO3] = ?
[IO3] = ?

) cchieh@uwaterloo.ca