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
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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)32x 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 = 1.173pH = 14 - pOH = 14 - 1.173 = 12.826
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