Homework Assignment #13

1) A solution of perchloric acid is prepared that has a molar concentration equal to the fourth and fifth digits of your Student ID Number divided by 105. What is the pH and pOH of that solution at 25 °C?

2) A solution of nitric acid is prepared that has a molar concentration equal to the fourth and fifth digits of your Student ID Number divided by 104. What is the pH and pOH of that solution at 25 °C?

3) A solution of potassium hydroxide is prepared that has a molar concentration equal to the fourth and fifth digits of your Student ID Number divided by 103. What is the pH and pOH of that solution at 25 °C?

4) A solution of barium hydroxide is prepared that has a molar concentration equal to the fourth and fifth digits of your Student ID Number divided by 105. What is the pH and pOH of that solution at 25 °C? Assume complete solubility and complete ionization.

Answers:

1) For perchloric acid:

HClO4(aq) + H2O(l) → H3O+(aq) + ClO4–(aq)

So the [H3O+] = [HClO4] = ID/105 M

pH = –log[H3O+]

pOH = 14.00 – pH (at 25 °C)

2) For nitric acid:

HNO3(aq) + H2O(l) → H3O+(aq) + NO3–(aq)

So the [H3O+] = [HNO3] = ID/104 M

pH = –log[H3O+]

pOH = 14.00 – pH (at 25 °C)

3) For potassium hydroxide:

KOH(aq) → K+(aq) + OH–(aq)

So [OH–] = [KOH] = ID/103 M

pOH = –log[OH–]

pH = 14.00 – pOH (at 25 °C)

4) For barium hydroxide:

Ba(OH)2(aq) → Ba2+(aq) + 2 OH–(aq)

So [OH–] = 2×[Ba(OH)2] = 2×ID/105 M

pOH = –log[OH–]

pH = 14.00 – pOH (at 25 °C)

Since all concentrations are given to 2 significant figures, all pH and pOH values are found to 2 places past the decimal.

Specific answers:

Question 1 Question 2 Question 3 Question 4
[HClO4] (M) pH pOH [HNO3] (M) pH pOH [KOH] (M) pH pOH [Ba(OH)2] (M) pH pOH
0.00050 3.30 10.70 0.0050 2.30 11.70 0.050 12.70 1.30 0.00050 11.00 3.00
0.00052 3.28 10.72 0.0052 2.28 11.72 0.052 12.72 1.28 0.00052 11.02 2.98
0.00054 3.27 10.73 0.0054 2.27 11.73 0.054 12.73 1.27 0.00054 11.03 2.97
0.00055 3.26 10.74 0.0055 2.26 11.74 0.055 12.74 1.26 0.00055 11.04 2.96
0.00056 3.25 10.75 0.0056 2.25 11.75 0.056 12.75 1.25 0.00056 11.05 2.95
0.00057 3.24 10.76 0.0057 2.24 11.76 0.057 12.76 1.24 0.00057 11.06 2.94
0.00058 3.24 10.76 0.0058 2.24 11.76 0.058 12.76 1.24 0.00058 11.06 2.94
0.00059 3.23 10.77 0.0059 2.23 11.77 0.059 12.77 1.23 0.00059 11.07 2.93
0.00060 3.22 10.78 0.0060 2.22 11.78 0.060 12.78 1.22 0.00060 11.08 2.92
0.00061 3.21 10.79 0.0061 2.21 11.79 0.061 12.79 1.21 0.00061 11.09 2.91
0.00062 3.21 10.79 0.0062 2.21 11.79 0.062 12.79 1.21 0.00062 11.09 2.91
0.00063 3.20 10.80 0.0063 2.20 11.80 0.063 12.80 1.20 0.00063 11.10 2.90