Question

A solution is 5 mM in each of the following ions:

number	ion	Ksp of M(OH)2
1	Mg2+	1.8e-11
2	Cd2+	2.5e-14
3	Co2+	1.6e-15
4	Zn2+	4.4e-17
5	Cu2+	2.2e-20

Indicate which of the metal ions would precipitate (or start to precipitate) at each of the following pH values. Indicate your answer with the number of the ion. Use 0 to indicate no precipitate. If more than one precipitate is expected, list the numbers in increasing order and separate them with commas. For example, 3,4,5 is ok but 5,4,3 is not.

pH = 5.00:

pH = 10.00:

What is the pH to the nearest 0.1 pH unit at which Zn(OH)₂ begins to precipitate?

pH =

Answer 1

At pH= 5 would precipitate

We need to figure out if K is bigger than Q for each of these reactions...if K < Q, then precipitation will occur, and if K > Q, it won't.

So Q for these ions = [ion][OH1-]^2, because prod/reac, the reactant is a solid so we don't worry about it, and the OH term is squared because there are 2 OHs in M(OH)2

[ion] we have because each sol'n is 5mM. [OH] we have, because we have the pH. (7 and 8, in this case.)

So you pretty much go through and plug in your numbers, into the Q=[ion][OH1-] equation. Q for each term is the same, since each ion has the same concentration at each pH, (so there's a Q value at pH 7, and one at pH 8) and you basically go through and compare them to the Ksp values given to us and figure out if each individual one precipitates.

For 3, Q has to = K, so just set that same equation above = Ksp for Co. You now know Ksp and [Co], so you can find [OH] and then the pH.

Then

* At pH 5.0 would precipitate : 4,5

*At pH 10 would precipitate 3,4,5