A solution is 5 mM in each of the following ions: Indicate which of the metal io
ID: 507150 • Letter: A
Question
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.
number ion KS 1.8e-11 Indicate which af the metal ions would precipitate (or start to precipitate) at each of the fallowing pH values. Indicate yaur answer with the number of the ion. use 0 ta indicate no precipitate. If more than one precipitate is expected list the numbers in increasing th commas. For example, 3 4,5 is ok b 5,4,3 is order and separate them not. pH 5.00: pH 9.00Explanation / Answer
Part A
The pH of the solution is 5.00. Since pH = 5.00, we must have pOH = 14.00 – pH = 14.00 – 5.00 = 9.00 (since pH + pOH = 14).
Again, pOH = -log [OH-]
===> [OH-] = antilog (-pOH) = antilog (-9.00)
===> [OH-] = 1.0*10-9
The concentration of hydroxide ion is 1.0*10-9 M.
Note that all the salts contain divalent metal cation with monovalent OH- ion. The dissociation reaction is
M(OH)2 = M2+ (aq) + 2 OH- (aq)
The reaction quotient is given as Q = [M2+][OH-]2. A precipitate will occur when Q > Ksp. When Q = Ksp, the solution is just saturated and the precipitate begins to form. When Q < Ksp, no precipitate forms.
Construct the following table. Note that the concentration of the metal ions = 5 mM = 5.0*10-3 M.
Number
Ion
Q = [M2+][OH-]2
Whether precipitate forms or not?
1
Mg2+
(5.0*10-3)*(1.0*10-9)2 = 5.0*10-21
No, because Q is less than Ksp
2
Cd2+
(5.0*10-3)*(1.0*10-9)2 = 5.0*10-21
No, because Q is less than Ksp
3
Co2+
(5.0*10-3)*(1.0*10-9)2 = 5.0*10-21
No, because Q is less than Ksp
4
Zn2+
(5.0*10-3)*(1.0*10-9)2 = 5.0*10-21
No, because Q is less than Ksp
5
Cu2+
(5.0*10-3)*(1.0*10-9)2 = 5.0*10-21
No, because Q is less than Ksp
At pH 5.00, none of the metal ions will form precipitates.
Ans: 0
Part B
The pH of the solution is 9.00. Since pH = 9.00, we must have pOH = 14.00 – pH = 14.00 – 9.00 = 5.00.
Again, pOH = -log [OH-]
===> [OH-] = antilog (-pOH) = antilog (-5.00)
===> [OH-] = 1.0*10-5
The concentration of hydroxide ion is 1.0*10-5 M.
Note that all the salts contain divalent metal cation with monovalent OH- ion. The dissociation reaction is
M(OH)2 = M2+ (aq) + 2 OH- (aq)
Construct the following table. Note that the concentration of the metal ions = 5 mM = 5.0*10-3 M.
Number
Ion
Q = [M2+][OH-]2
Whether precipitate forms or not?
1
Mg2+
(5.0*10-3)*(1.0*10-5)2 = 5.0*10-13
No, because Q is less than Ksp
2
Cd2+
(5.0*10-3)*(1.0*10-5)2 = 5.0*10-13
Yes, because Q exceeds Ksp
3
Co2+
(5.0*10-3)*(1.0*10-5)2 = 5.0*10-13
Yes, because Q exceeds Ksp
4
Zn2+
(5.0*10-3)*(1.0*10-5)2 = 5.0*10-13
Yes, because Q exceeds Ksp
5
Cu2+
(5.0*10-3)*(1.0*10-5)2 = 5.0*10-13
Yes, because Q exceeds Ksp
All the metal ions except Mg2+ will form precipitate with OH- at pH 9.00
Ans: 2, 3, 4, 5
Number
Ion
Q = [M2+][OH-]2
Whether precipitate forms or not?
1
Mg2+
(5.0*10-3)*(1.0*10-9)2 = 5.0*10-21
No, because Q is less than Ksp
2
Cd2+
(5.0*10-3)*(1.0*10-9)2 = 5.0*10-21
No, because Q is less than Ksp
3
Co2+
(5.0*10-3)*(1.0*10-9)2 = 5.0*10-21
No, because Q is less than Ksp
4
Zn2+
(5.0*10-3)*(1.0*10-9)2 = 5.0*10-21
No, because Q is less than Ksp
5
Cu2+
(5.0*10-3)*(1.0*10-9)2 = 5.0*10-21
No, because Q is less than Ksp
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