Chemistry Lab

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
&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.

  1. 👍
  2. 👎
  3. 👁

Respond to this Question

First Name

Your Response

Similar Questions

  1. Chemsitry

    The Ksp of PbBr2 is 6.60× 10^–6. What is the molar solubility of PbBr2 in pure water? What is the molar solubility of PbBr2 in 0.500 M KBr solution? What is the molar solubility of PbBr2 in a 0.500 M Pb(NO3)2 solution?

  2. Chemistry

    (Ksp= 5.0*10^-13) 1) Calculate the molar solubility of AgBr in 3.0×10^−2 M AgNO3 solution. 2) calculate the molar solubility of AgBr in 0.10 M NaBr solution. I tried solving using this the quadratic formula, but it didn't work.

  3. Chemistry

    If two salts AX and BX2, have the same Ksp values of 4.0x10^-12 at a given temperature, then A. their molar solubilities in water are the same B. the salts are more soluble in 0.1 M NaX than in water C. the molar solubility of AX

  4. Chemistry

    Using the following information, identify the strong electrolyte whose general formula is Mx(A)y * zH2O Ignore the effect of interionic attractions in the solution. a. A"-is a common oxyanion. When 30.0 mg of the anhydrous sodium

  1. Chemistry

    Which of the following compounds has the greatest molar solubility? A) AgBr, Ksp= 5.4 x 10^13 B) Ba3(PO4)2, Ksp = 3.0 x 10^-23 C) Al(OH)3, Ksp = 1.9 x 10^-33 D) MgF2, Ksp = 7.4 x 10^-11 E) Pb(OH)2, Ksp = 1.2 x 10^-15 I know the

  2. chemistry

    Suppose that you wanted to be sure that a metal ion, any metal ion, would dissolve in water. What salt of the metal ion compound would you choose? 1. the hydroxide (OH−) salt of the metal ion 2. the carbonate (CO2− 3 ) salt of

  3. Chemistry

    The solubility of insoluble substances changes depending on the nature of the solution. Below are two solutions in which Cu(OH)2 is dissolved; in each case, the solubility of Cu(OH)2 in those solutions is not the same as it is in

  4. Chemistry

    Determine the molar solubility of MX (Ksp=4.2x10-8) in 0.047 M NaCN. The metal ion M+ forms M(CN)2- (Kf=3.1x1010) in the presence of CN- ion

  1. Chemistry

    generic salt, AB3, has a molar mass of 333 g/mol and a solubility of 2.30 g/L at 25 °C. What is the Ksp of this salt at 25 °C?

  2. chem

    The solubility product of nickel (II) hydroxide at 25°C is 1.6 x 10-16. a) Calculate the molar solubility of nickel (II) hydroxide in pure water at 25°C. b) Calculate the molar solubility of nickel (II) hydroxide in 0.100 M

  3. Chemistry 102

    The Ksp for cerium iodate, Ce(IO3)3 is 3.2 e-10. What is the molar solubility of Ce ion in pure water? = .0019 M B) A 0.031 M sodium iodate is added as a common ion. What is the concentration of Ce ion with the common ion present?

  4. Chemistry

    an unknown salt is either NaCN,NaC2H3O2, NaF, NaCl, or NaOCl. when 0.100 molar of the salt is dissolved in 1.00 L of solution, the ph of the solution is 8.07. what is the identity of the salt? where do I start? How do I solve it,

You can view more similar questions or ask a new question.