The pH of Mistures of Acid, Base, and Salt Solutions a. When 0.10 mol of the ion
ID: 908481 • Letter: T
Question
The pH of Mistures 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 1.0L of solution, the pH of the solution is 9.12. When 0.10 mol of the ionic solic ACl, where A in an unknown cation, is dissolved in enough water to make 1.0L solution, the pH of the solution is 7.00. What would be the pH of 1.0L 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 [H30+].
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 1.0L 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 1.0L 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 1.0L sample of solution that contains 0.10 mol of AX and 0.10 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.
Explanation / Answer
a. Mixing NaX and ACl would result in a basic solution.
X- is a stronger base say CH3COO- would react with water to form OH- in solution. This would ,ake the solution basic. In AX the same X- acts to give OH- in solution. In ACl, both A and Cl are coming from stroing acid and strong base say NaOH and HCl so the resultant salt in water would give a neutral pH = 7.
b. With AX in solution we have OH- in solution. the concentration of OH- at any time would be greater than H3O+ in solution.
c. Kb for X- would be = [HX][OH-]/[X-]
and,
Ka for conjugate acid of HX would be,
Ka = Kw/Kb = [X-]/[HX][OH-]
d. If we to add 0.025 M HCl to the AX solution. the pH of solution would drop by the acid added. X- in solution would react with the added H+ to form HX. The solution still have excess of AX in solution, so the pH still be would be basic. As compared to NaX the pH would be lower than NaX.
e. When you add 0.1 mol of HCl, the mols of X- equals the mols of HCl added. So the pH has dropped significantly. Now we only have HX in the system which dissociated to give H+ and thus pH is acidic.
f. When you add 0.1 M NaOH to a 0.1 M in 1L solution. The added OH- would react with water in the system to produce more OH-. So overall pH would be higher. More OH- less H+, higher pH.
g. The student might be thinking that all of HX formed from 0.1 M AX and 0.1 M HCl here would dissociate to give 0.1 mol of H+. This would thus give a pH of 1.
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