NS FL3 C/P #51

[NS_Tutor_Will]

A student recently asked, "Was there a mistake in option A? It says the Ksp for silver sulfate is much larger than the Ksp for the yellow percipitate, however, the yellow percipitate IS silver sulfate!"

In paragraph two of this passage, we are told that mass spectrometry performed on the yellow precipitate showed two absorption peaks which correlated with a phosphate group and silver. Thus, the yellow precipitate must be silver phosphate (Ag3PO4).

The Ksp of silver sulfate is much larger than the Ksp of silver phosphate, so we wouldn't expect silver sulfate to precipitate.

I hope this helps!

[moljaca3]

So I was able to get the correct answer through POE, but the correct answer doesn't make a whole lot of sense to me.

I understand that the yellow precipitate is Ag3PO4, but I don't get how the Ksp of silver sulfate (Ag2SO4) is very large compared to the Ksp of Ag3PO4.
From my understanding, the Ksp for Ag3PO4 would be Ksp={Ag+]^3[PO43-], while the Ksp for Ag2SO4 would be Ksp=[Ag+]^2[SO42-]. Doesn't this make the Ksp of Ag3PO4 larger because [Ag+] is raised to the 3rd power versus to the 2nd power for Ag2SO4?

[NS_Tutor_Will]

I think I see what you're saying. The trick here is that we're looking for something that might explain the results. The answer explanation touches on this well:

A large Ksp suggests that a substance is more soluble than a substance with a lower Ksp. It is not certain, given that the coefficients might be different and might affect the power to which the ion concentrations are raised. However, it provides a better possible explanation than the other answer choice options (and the phrasing of the question prompt, using the words “might explain,” notes that there is some uncertainty involved).

So part of this is that this could make sense! We aren't required to memorize Ksp values for substances (fortunately), but remember these are experimentally derived and are equilibrium expressions. So, just because an expression is raised to a large power, doesn't necessarily mean that the Ksp value will be higher.

In this case, the correct answer choice is a potential explanation that does mesh with the data, while the other options do not.

[moljaca3]

Let me see if my reasoning is correct:

Since we aren't required to memorize Ksp's, it is possible that at equilibrium, silver sulfate is dissolved to a much greater extent than silver phosphate. So even though the Ksp for silver phosphate is raised to a larger power than that of silver sulfate, the Ksp for silver sulfate could be larger than that of silver phosphate once you plug in the equilibrium concentrations.

Does that make sense?

[NS_Tutor_Will]

Yep, that makes perfect sense!