Concept explainers
(a)
Interpretation:
The initial pH value of the weak base before adding the acid is to be determined.
Concept introduction:
A strong acid is the substance which completely dissociates into hydrogen ion and conjugate base in water. The weak base partially dissociate into its respective ions.
(a)
Answer to Problem 167AE
The pH of the initial weak base solution is 9.67.
Explanation of Solution
Initial pH of the analyte solution;
The amount of weak base at the beginning
Reaction | Weak base | Conjugate acid | OH- |
Initial | 0.2 | 0 | 0 |
Change | -x | +x | +x |
Equilibrium | (0.2-x) | x | x |
For small base dissociation constant value, the value of x can be neglected from the denominator.
Thus,
Then the pH of the initial solution can be determined.
(b)
Interpretation:
The pH value of the weak base after adding the acid is to be determined.
Concept introduction:
A strong acid is the substance which completely dissociates into hydrogen ion and conjugate base in water. The weak base partially dissociate into its respective ions.
(b)
Answer to Problem 167AE
The pH of the solution after adding the acid is 6.88.
Explanation of Solution
Addition of
Total amount of base to be neutralized
Amount of base added
Then the ICE table after the addition of base is created in order to determine the pH of the solution using Henderson-Hasselbalch equation.
Reaction | Weak base | H+ | Conjugate acid | OH- |
Initial | 0.02 | 0 | 0 | 0 |
Add | 0 | 0.0025 | ||
Change | -0.0025 | -0.0025 | 0.0025 | 0.0025 |
Equilibrium | 0.0175 | 0 | 0.0025 | 0.0025 |
The base dissociation constant value is
Thus,
Since, volume is same thus, number of moles can be used instead of concentrations.
Applying the Henderson-Hasselbalch equation,
Now, pH can be calculated as follows:
Thus, the pH of the solution is 6.88.
(c)
Interpretation:
The pH value of the weak base after adding the acid (70 mL HCl) is to be determined.
Concept introduction:
A strong acid is the substance which completely dissociates into hydrogen ion and conjugate base in water. The weak base partially dissociate into its respective ions.
(c)
Answer to Problem 167AE
The pH of the solution after adding the acid is 6.31.
Explanation of Solution
Addition of
Total amount of base to be neutralized
Amount of base added
Then the ICE table after the addition of base is created in order to determine the pH of the solution using Henderson-Hasselbalch equation.
Reaction | Weak base | H+ | Conjugate acid | OH- |
Initial | 0.0200 | 0 | 0 | 0 |
Add | 0 | 0.0070 | ||
Change | -0.0070 | -0.0070 | 0.0070 | 0.0070 |
Equilibrium | 0.0130 | 0 | 0.0070 | 0.0070 |
Applying the Henderson-Hasselbalch equation,
Thus, the pH of the solution is 6.31.
(d)
Interpretation:
The pH value of the weak base at equivalent point needs to be determined.
Concept introduction:
At equivalent point the concentration of acid and base (in which one of the species is weak) is equal in the solution. Thus, the pH will depend on the acidic or basic salt formed.
(d)
Answer to Problem 167AE
The pH of the solution at the equivalence point is 3.60.
Explanation of Solution
At the equivalence point, The amount of acid added
The volume of acid added
At this point, there is no excess acid or base. Therefore, the only possible reaction here is the dissociation of the conjugate acid of the ammonia (that is ammonium ion).
Thereafter, using the Ka value for this weak base, the amount of hydrogen ions in the solution can be determined to get the pH value at this point.
Now, total volume of the solution is 300 mL thus, molarity of HONH3+ solution will be:
Reaction | HONH3+ | HONH2 | H+ |
Initial | 0.0667 | 0 | 0 |
Change | -x | x | x |
Equilibrium | (0.0667-x) | x | x |
Then the pH can be calculated as follows:
This value is equal to the amount of hydrogen ions in the solution.
Thus, the pH value at equivalence point is 3.60.
(e)
Interpretation:
The pH value of the weak base after adding the acid (300 mL) is to be determined.
Concept introduction:
A strong acid is the substance which completely dissociates into hydrogen ion and conjugate base in water. The weak base partially dissociate into its respective ions.
(e)
Answer to Problem 167AE
The pH of the solution after adding acid is
Explanation of Solution
Addition of
At this point, there is excess acid in the solution, thus, the pH can be calculated as follows:
Excess amount of acid in the solution
The amount of hydrogen ions
The concentration of hydrogen ions
(f)
Interpretation:
The volume of HCl needs to be determined that is added to the solution if pH is 6.04.
Concept introduction:
A strong acid is the substance which completely dissociates into hydrogen ion and conjugate base in water. The weak base partially dissociate into its respective ions.
(f)
Answer to Problem 167AE
100 mL
Explanation of Solution
The
The Henderson Hesselbalch equation can be represented as follows:
Since, the volume is same thus, concentration can be replaced by number of moles.
Thus, the number of moles of salt and base will be same that is
This is the condition which is halfway to the equivalence point. For the equivalence point, volume of HCl required was 200 mL thus, at halfway equivalence point the volume will be half to it. Therefore, volume of HCl is 100 mL.
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Chapter 8 Solutions
Chemical Principles
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