Question about Ksp/Solubility

holler01 · Aug 29, 2008

This is from Kaplan PS Section Test 2:

I was a little confused after picking the opposite answer. To my understanding, a higher Ksp indicates that more of the ion can be added before precipitation occurs, but I guess im wrong?

The question asked: 1M NaOH is added to the following solutions (each is 1M) which will precipitate first?
AgOH (Ksp = 1.5 x 10 ^-8)
Al(OH)3 (ksp = 3.7 x 10^-15)
Mg(OH)2 (ksp = 1.2 x 10 ^-11)
Mn(OH)2 (ksp = 2 x 10^ -13)

I picked Al(OH)3 because i thought a lower ksp meant it would precipitate first, but im wrong. The answer is AgOH.

can anyone explain?!

161927 · Aug 29, 2008

holler01 said:
This is from Kaplan PS Section Test 2:

I was a little confused after picking the opposite answer. To my understanding, a higher Ksp indicates that more of the ion can be added before precipitation occurs, but I guess im wrong?

The question asked: 1M NaOH is added to the following solutions (each is 1M) which will precipitate first?
AgOH (Ksp = 1.5 x 10 ^-8)
Al(OH)3 (ksp = 3.7 x 10^-15)
Mg(OH)2 (ksp = 1.2 x 10 ^-11)
Mn(OH)2 (ksp = 2 x 10^ -13)

I picked Al(OH)3 because i thought a lower ksp meant it would precipitate first, but im wrong. The answer is AgOH.

can anyone explain?!

Ksp(AgOH) = [Ag+][OH]
c.f. Ksp(Al(OH)3) = [Al3+][OH-]^-3

the exponents must be considered as part of the equilibrium expression. That Al(OH)3 has a lower Ksp is misleading, because this resulted from different stoichiometric concentrations. AgOH is the best answer, because when you solve for "s", the Ksp is lowest.

Kaustikos · Aug 30, 2008

DCODY said:
Ksp(AgOH) = [Ag+][OH]
c.f. Ksp(Al(OH)3) = [Al3+][OH-]^-3

the exponents must be considered as part of the equilibrium expression. That Al(OH)3 has a lower Ksp is misleading, because this resulted from different stoichiometric concentrations. AgOH is the best answer, because when you solve for "s", the Ksp is lowest.

Yeah. It's these "trick" questions you need to watch out for. Kaplan loves to ask questions like these, which is good, because it helps you get a better understanding of what the Ksp means.

mosquitoman · Sep 1, 2008

DCODY said:
Ksp(AgOH) = [Ag+][OH]
c.f. Ksp(Al(OH)3) = [Al3+][OH-]^-3

the exponents must be considered as part of the equilibrium expression. That Al(OH)3 has a lower Ksp is misleading, because this resulted from different stoichiometric concentrations. AgOH is the best answer, because when you solve for "s", the Ksp is lowest.

i really don't understand your explanation.. could you elaborate?

for AgOH S^2 = 1.5 x 10 ^-8
S ~= 10^-4

for Ag(OH)3, s^4 = 3.7 x 10^-15
S ~= 2x10^-4

so isnt AgOH3 more soluble? or is this due to common ion effect where Ag(OH)3 produces the most OH ?

161927 · Sep 1, 2008

mosquitoman said:
i really don't understand your explanation.. could you elaborate?

for AgOH S^2 = 1.5 x 10 ^-8
S ~= 10^-4

for Ag(OH)3, s^4 = 3.7 x 10^-15
S ~= 2x10^-4

so isnt AgOH3 more soluble? or is this due to common ion effect where Ag(OH)3 produces the most OH ?

Ag(OH)3
(s)(3s)^3 = 27s^4

AWhitehair · Sep 2, 2008

mosquitoman said:
i really don't understand your explanation.. could you elaborate?

for AgOH S^2 = 1.5 x 10 ^-8
S ~= 10^-4

for Ag(OH)3, s^4 = 3.7 x 10^-15
S ~= 2x10^-4

so isnt AgOH3 more soluble? or is this due to common ion effect where Ag(OH)3 produces the most OH ?

First off the red above SHOULD be Al, not Ag. To answer your question, yes, AlOH3 is more soluble, but that is not the answer because the question is which one will precipitate first...thus, the least soluble one will ppt first, which is AgOH.

junkct · Sep 3, 2008

AWhitehair said:
First off the red above SHOULD be Al, not Ag. To answer your question, yes, AlOH3 is more soluble, but that is not the answer because the question is which one will precipitate first...thus, the least soluble one will ppt first, which is AgOH.

Exactly. Basically you can translate the question to "Which has the lowest solubility?"

AgOH --> Ag+ + OH-
Ksp = [Ag+][OH-] = (s)(s) = s^2

Al(OH)3 --> Al3+ + 3OH-
Ksp = [Al3+][3OH-]^3 = (s)(27s^3) = 27s^4

Plug in for Ksp and you'll see that 's' for AgOH is < 's' for Al(OH)3, meaning the solubility is lower and it will precipitate first.

Question about Ksp/Solubility

holler01

Full Member

161927

Kaustikos

Antibiotics 4 Lyfe

mosquitoman

Full Member

161927

AWhitehair

EM PA-C. MD wannabe

junkct

Full Member

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