Analytical Chemistry - pH

Chemistry
Tutor: None Selected Time limit: 1 Day

An acidic solution containing 0.01 M La +3 is treated with NaOH until La(OH)3 begins to precipitate. At what pH does this occur? Ksp for La(OH)3 is 2 x 10 -21

Sep 30th, 2015

Thank you for the opportunity to help you with your question!

Write the dissociation equation:

La(OH)3 <===> La3+ + 3OH¯

2) Write the Ksp expression:

Ksp = [La3+] [OH¯]3

3) Plug into the Ksp expression:

2x 10¯21 = (x) (3x)3

2x 10¯21 = (0.01) (3x)3

4) Solve for [OH¯]:

[Cr3+] = x =  4.05 * 10^ -7 M (I kept some guard digits)
[OH¯] = 3x = 1.216 x 10¯ 6M

5) Calculate the pH given the [OH¯]:

pOH = -log 1.216 x 10¯ 6 = 1.173
pH = 14 - pOH = 14 - 1.173 = 12.826

Please let me know if you need any clarification for the doubts. I'm always happy to answer your questions. Looking forward to help you again. thank you. :)
Sep 30th, 2015

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