Cliffs AP Chemistry, 3rd Edition

8. Calculate the theoretical pH of the distilled water and added HCl (beaker 3).
   ..^2
   HLL (.. ).
   mol H
   0 0250 L M
   0 250
   0 0250 0 0500
   = ###^1 =833 10

+ + -

7A

pH = −log [H+] = −log (8.33 × 10 –2) = 1.08

9. Calculate the theoretical pH of the distilled water and added NaOH (beaker 4).
   ..^2
   OH L L (.. ).
   mol OH
   0 0250 L M
   0 250
   0 0250 0 0500
   ==###^1 833 10

7A

pOH = −log [OH–] = −log (8.33 × 10 –2) = 1.08

pH = 14.00 −pH = 14.00 −1.08 = 12.92

10. A 0.1 M solution of an acid was tested with the following indicators and the following
    colors observed:
    Phenolphthalein: clear Bromthymol blue: blue
    Thymol blue: yellow Alizarin yellow: yellow
    Methyl orange: yellow Methyl red: yellow

Using Appendix B, what is the approximate pH of the solution? What two indicators would
you use to find the pH more precisely and why?

From the indicators that were used, the following is a summary of their properties:

Phenolphthalein: range 8.3 to 10.0 Lower color-colorless; Upper color-dark pink

Bromthymol blue: range 6.0 to 7.6 Lower color-yellow; Upper color-blue

Thymol blue: range 8.1 to 9.5 Lower color-yellow; Upper color-blue

Alizarin yellow: range 9.9 to 11.8 Lower color-yellow; Upper color-dark orange

Methyl orange: range 2.5 to 4.4 Lower color-red; Upper color- yellow

Methyl red: range 4.4 to 6.2 Lower color-red; Upper color-yellow

m-Cresol purple: range 7.5 to 9.0 Lower color-yellow; Upper color-violet

Using Appendix B, you would choose bromthymol blue (range 6.0 to 7.6, Lower color-yellow;
Upper color-blue) and m-Cresol purple (range 7.5 to 9.0, Lower color-yellow; Upper color-
violet) because bromthymol blue’s high-end color (blue) ends where m-cresol purple’s low-end
color (yellow) begins. The pH would be between 7.5−8.5.

Laboratory Experiments