Chemistry Lab

posted by .

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.
|
|
|
|
|
|
|
|_______________________________

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.
|
14
|
13
|
12
|
11
|
10 pH
|
|
|
|
0-10-20-30-40-50-60-70-80-90-100
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.

Similar Questions

1. Phenolphtalein vs Methyl Orange (check my reasonin

Why can either phenolphthalein or methyl orange be used for an HCl-NaOH titration, but only phnolphthalein is suitable for an acetic acid-NaOH titration?
2. Chemistry - Acid Base Titration

When you complete a acid base titration of NaOH and HCl and how to you find the molarity/concentration of HCl. I used 9 mL of HCl with 50 mL of water. But when solving for concentration, do I use 9 mL or 59 mL (water + HCl)?
3. Chemistry

My teacher has us doing a virtual Titration lab on sciencegeek site but never demonstrated a Titration Lab so I really don't undestand it. We have to identify five unknown acids according to how much [H+} of acid is in each solution. …
4. Chemistry -help

To determine the sodium carbonate content & total alkali in NaOH, the following steps were performed:- 1) 2.0g of NaOH was dissolved in 80ml of CO2 free water. 2) 3 drops of phenolphthalein indicator were added and the solution was …
5. chemistry

This is from a "effect of pH on the solubility of Ca(OH)2" lab: If excess Ca(NO3)2 were present, what effect would it have on the solubility of Ca9OH)2?
6. chemistry lab

How does the presence of a common ion affect the solubility of a salt?
7. science

OBJECTIVE: to find out which of two indicators, methyl orange or phenolphthalein, gives more reliable and consistent results in the titration of H3PO4 with NaOH. REQUIREMENT: (1). Phosphoric acid solution of unknown concentration. …
8. Chemistry

For the titration of hcl versus NaOh, suggest a better indicator than methyl orange. Why is methyl orange not the ideal choice for this application?
9. science, chemistry

OBJECTIVE: to find out which of two indicators, methyl orange or phenolphthalein, gives more reliable and consistent results in the titration of H3PO4 with NaOH. REQUIREMENT: (1). Phosphoric acid solution of unknown concentration. …
10. Chemistry (molarity) (sorta an emergency)

I'm doing a titration lab and writing a lab report where I'm sort of stuck on how exactly to find the concentration of HCl. These are the following info I have from the lab, Equation: HCl(aq)+NaOH(aq)-> H2O(l)+NaCl(aq) -Calculated …

More Similar Questions