7. A solution is 5 mM in each of the following ions: Indicate which of the metal

Question

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

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.

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

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

Explanation / Answer

pH = 6 pOH = 8 so 10^-8 for OH^1- 5mM is .005M Solve for Q Q = [.005M] [10^-8]^2 Q = 5.0e-19 compare K and Q in alphabetical order K > Q No ppt K < Q Yes ppt answer is only 5 you do the same thing for pH of 8, i had a different number for this one, but should get Q of 5.0 e-15, then compare like in the first part. for part 2 solve K K = [Zn^2+][OH^1-] 4.4e-17 = (.005) [OH^1-]^2 solve for OH, i got 9.38e-8 -log (9.38e-8) gives pOH, then 14 - pOH to get pH my numbers were different except the first part so double check, but i did it the same way and got it right. Update: yes i got: 3,4,5 1. K 1.8e-11 > Q 5.0e-15 2. K 2.5e-14 > Q 3. K 1.6e-15 < Q 4. K 4.4e-17 < Q 5. K 2.2e-20 < Q when Q is greater than K, then there will be a precipitate.ffb86

Related Questions

Navigate

• Browse (All)
• Browse 7
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.