Concept explainers
The pH of Mixtures of Acid, Base, and Salt Solutions
- a When 0.10 mol of the ionic solid NaX, where X is an unknown anion, is dissolved in enough water to make 1.0 L of solution, the pH of the solution is 9.12. When 0.10 mol of the ionic solid ACl, where A is an unknown cation, is dissolved in enough water to make 1.0 L of solution, the pH of the solution is 7.00. What would be the pH of 1.0 L of solution that contained 0.10 mol of AX? Be sure to document how you arrived at your answer.
- b In the AX solution prepared above, is there any OH− present? If so, compare the [OH−] in the solution to the [H3O+].
- c From the information presented in part a, calculate Kb for the X−(aq) anion and Ka for the conjugate acid of X−(aq).
- d To 1.0 L of solution that contains 0.10 mol of AX, you add 0.025 mol of HCl. How will the pH of this solution compare to that of the solution that contained only NaX? Use
chemical reactions as part of your explanation; you do not need to solve for a numerical answer. - e Another 1.0 L sample of solution is prepared by mixing 0.10 mol of AX and 0.10 mol of HCl. The pH of the resulting solution is found to be 3.12. Explain why the pH of this solution is 3.12.
- f Finally, consider a different 1.0-L sample of solution that contains 0.10 mol of AX and 0.1 mol of NaOH. The pH of this solution is found to be 13.00. Explain why the pH of this solution is 13.00.
- g Some students mistakenly think that a solution that contains 0.10 mol of AX and 0.10 mol of HCl should have a pH of 1.00. Can you come up with a reason why students have this misconception? Write an approach that you would use to help these students understand what they are doing wrong.
(a)
Interpretation:
The steps for the calculation of pH of
Concept Introduction:
Salt hydrolysis:
Salt hydrolysis is a reaction in which the ion of salt reacts with water and produce either hydronium ion or hydroxide ion.
Based on pH of the solution, salt solutions can be classified as
- Acidic-(pH will be less than seven)
- Basic -(pH will be more than seven)
- Neutral -(pH will be equal to seven)
To Explain: The steps for the calculation of pH of
Answer to Problem 16.22QP
The pH of the solution prepared from the ionic compound
Explanation of Solution
Given data:
A
A
A
Calculation of pH of
The ionic compound
The ionic compound
The ionic compound
The pH of the solution prepared from the ionic compound
(b)
Interpretation:
Does the
Concept Introduction:
Salt hydrolysis:
Salt hydrolysis is a reaction in which the ion of salt reacts with water and produce either hydronium ion or hydroxide ion.
Based on pH of the solution, salt solutions can be classified as
- Acidic-(pH will be less than seven)
- Basic -(pH will be more than seven)
- Neutral -(pH will be equal to seven)
Answer to Problem 16.22QP
The
On comparison of concentration,
Explanation of Solution
Given data:
A
This solution is found be basic with pH of 9.12 (from part (a))
Since the
Since the solution is a basic solution, the concentration of hydroxide ions will be greater than the concentration of hydronium ions.
Thus,
The
On comparison of concentration,
(c)
Interpretation:
Using part (a) information, the
Concept Introduction:
Relationship between
Where
To Calculate: Using part (a) information, the
Answer to Problem 16.22QP
The
Explanation of Solution
Given data:
A
This solution is found be basic with pH of 9.12 (from part (a))
Calculation of
A
Thus,
The hydrolysis of
The
Therefore,
The
The
The
(d)
Interpretation:
The pH of the
To Compare: The pH of the
Explanation of Solution
Given data:
A
A
A
This solution is basic and has a pH of 9.12
Comparison of pH:
On addition of
Since the
Since the solution of
The pH of the
(e)
Interpretation:
The pH of the
To Explain: The pH of the
Explanation of Solution
Given data:
A
The pH of this solution is 3.12
pH calculation:
Here, equal moles of strong acid and conjugate base are present, so the solution is equivalent to a solution of
Since
The pH of the
(e)
Interpretation:
The pH of the
Concept Introduction:
The relationship between pH and pOH is gives as,
To Explain: The pH of the
Explanation of Solution
Given data:
A
The pH of this solution is 13.00
pH calculation:
The given solution has a strong base (Sodium hydroxide) and a weak conjugate base,
The solution is dominated by the strong base.
Since the concentration of
Thus,
The pH is calculated from pOH as follows,
The pH of the
(g)
Interpretation:
The wrong assumption of pH as 1.00 for a solution containing
To explain: The wrong assumption of pH as 1.00 for a solution containing
Explanation of Solution
Given data:
Some students mistakenly think that a solution containing
Explanation for wrong assumption of pH:
This is a solution of equal moles of a strong acid and a weak base.
The students misconception makes them think that the strong acid dominates the behaviour, which is not the real case.
The acid and the base will react with each other and produce an equal number of moles of conjugate acid
Since the conjugate acid is a weak acid, you would not expect it to dissociate completely, so the pH would not be 1.00, which would be the case if it were a strong acid.
The wrong assumption of pH as 1.00 for a solution containing
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Chapter 16 Solutions
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