Sample Exercise 17.17 - Practice Exercise #2 error

[Zellmer, Robert]

There's in error in the answer in the back of the book for Practice
Ex. #2 that follows Sample Exercise 17.17.

The problem deals with precipitating Mg(OH)2 and Cu(OH)2 from
a soln of 0.050 M Mg^2+ and 0.050 M Cu^2+ by adding OH-.
It wants to know which ppt first and the conc. necessary to ppt
each.

You use the Ksp values for Mg(OH)2 and Cu(OH)2 and the conc.
of the Mg^2+ and Cu^2+ to determine the [OH-] required to ppt
both, technically the equilibrium conc of OH- in a saturated soln.

Doing this you get the following:

For Mg(OH)2:

For a saturated soln [OH-] = 1.9 x 10^-5 M.  Thus, to get
Mg(OH)2 to ppt [OH-] > 1.9 x 10^-5 M.

For Cu(OH)2:

For a saturated soln [OH-] = 9.8 x 10^-10 M.  Thus, to get
Cu(OH)2 to ppt [OH-] > 9.8 x 10^-10 M.

This means Cu(OH)2 will ppt first.

This is the answer in the back of the book (Backmatter).  However,
they have the wrong conc. for the OH- required to begin ppt the
Cu(OH)2.

Dr. Zellmer

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