Question

What is a "common-ion effect", and how does it affect the molar solubility of a salt?




Why can either phenolphtalein or methyl orange be used for an HCl-NaOH titration, but only phenophtalein is suitable for an acetic acid-NaOH titration?


The answer to your first question is answered in your first post "To DrBob." The second one is hard to explain without a graph but I'll try. Here is what you need to do. On a sheet of paper, make a rudimentary set of y and x axes like so.
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Now, on the y axis, mark the origin as zero, and put dashes at 1, 2, 3, 4, 5, etc until you get to 14. Label this axis as pH. On the x axis, mark off units of 10 such as 10, 20, 30, 40, 50, etc until you get to 100. The x axis is marked as volume. The completed "incomplete" graph would look somthing like this.
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10 pH
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0-10-20-30-40-50-60-70-80-90-100
&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp volume in mL

Now. Starting at 0 mL and pH 1 draw a rather straight line more or less parallel to the x axis until you get to 50 mL. Make a smooth curve there, upward and continue parallel to the y axis until you get to a pH of 13, make another smooth curve downward (to the right) and continue the line parallel to the x axis. Label that curve the HCl vs NaOH curve. For the acetic acid/NaOH curve, Start at volume of zero mL and a pH of about 3.5, mark a point at 4 mL about pH = 4, mark a point at 45 mL and a pH = 5, mark a point at 49.5 mL and pH = 6, at 50 mL a pH = 9, at 50.5 mL a pH =11 55 mL a pH = 11.5 and at 100 mL pH = 12.5 . I haven't calculated these; rather I have guessed from memory. Connect these dots and label it the curve for acetic acid and NaOH. Now lets look where the indicators change color. Draw a band on the curve covering pH = 3.1 to 4.4 and label that methyl orange. Draw a band on the curve covering pH = 8.3 to 10.0. Label that phenolphthalein. With due respect for the crude way in which this was done, with estimating the volume/pH readings, and your drawing skills for a guesstimated graph, do you see that the vertical part of the curve for the HCl/NaOH starts about 4 or so and continues vertically until we get to about pH = 11 or so. That vertical portion covers both the area in which methyl orange changes as well as the area in which phenolphathalein changes. BUT that is not so for the acetic acid/NaOH curve. The methyl orange curve starts changing long before the vertical part of the curve so it is a slow slow slow change over and one would not be able to see a sharp change with just one or two drops of a titrant. Phenolphthalein, however, changes where the acetic acid/NaOH titration curve is vertical and there the change will be very sharp. We want titrations to be sharp, the end point must change within a drop or two (a half-drop is even better). Most indicators have a range of approximately 2 pH units from one color to the other color and we want that 2 pH unit change to be in the region where the titration curve is changing rapidly; i.e., where the pH is changing at least two pH units per 1 drop of titrant. I hope this helps. Let me know if I need to address any part of this. It would be better if I could have drawn a graph but perhaps this will do. I hope so.