E-Book Self-Test Question

[Patricia Sourinphoumy Dis1C]

I know that in Outline 1, it says to omit 5G.3, but this Self-test question fell under the 5G.2 reading, I believe...I decided to ask anyway.

Before this question came up, the text says, "A final point is that, when a reaction involves fully dissociated ionic compounds in solution, the equilibrium constant should be written for the net ionic equation by using the activity for each type of ion. The concentrations of the spectator ions cancel and so do not appear in the equilibrium expression."

Can someone explain the above statements? How do I know what ions are spectator ions?

And finally, here is the self-test question I was confused about and the given solution. I don't know how they got the expression for K:

Self-test 5G.3A Write the equilibrium constant for the reaction
2 AgNo3(aq) + 2 NaOH(aq) Ag2O(s) + 2 NaNO3(aq) + H2O(l)

Answer: K= 1/[Ag+]^2[OH-]^2

[Michelle Yeung 3I]

Spectator ions are usually in group I or II or are halogens. They got that expression for K because spectator ions do not affect pH so they leave them out of the equation. In this case, Na and NO3 are spectator ions since they do not react or affect pH. Per usual, H2O is not included because it is in excess. Hope this helps!