Find pH of 0.20 M Iron (III) Sulfate

[Qube]

Homework Statement

Find the pH of 0.20 M Iron (III) sulfate.

Homework Equations

The metal ion becomes hydrated (water is a ligand) and forms iron hexahydrate.

The sulfate ion is a weak base.

The Attempt at a Solution

Now, in considering the pH of this solution, should I consider the below reaction even though it's small extent? This reaction consumes the acid, but the reaction extent is small!

$Fe(OH_{2})_{6}^{3+} + SO_{4}^{2-} \leftrightharpoons HOSO_{2}^{-} + Fe(OH_{2})_{5}(OH)^{2+}$

Or should I ignore that reaction and only consider the below reaction?

$Fe(OH_{2})_{6}^{3+} + H_{2}O \leftrightharpoons H_{3}O^{+} + Fe(OH_{2})_{5}(OH)^{2+}$

 

[Borek]

Question is slightly ambiguous.

When you dissolve Fe³⁺ some of it will precipitate as Fe(OH)₃, substantially lowering solution pH (check Ksp). Question doesn't say whether 0.20 M was the initial concentration of the sulfate, or is the final concentration after the precipitation. Not that the difference will be large.

 

[Qube]

Good idea. We haven't covered solubility products yet; that's the next chapter. I'll just ignore precipitation but that is a valid concern. The 0.2M is referring to initial concentration of sulfate.

However, is the first, small extent reaction worth considering?

 

[Borek]

I would ignore sulfate and concentrate on the Fe³⁺ behavior, as it is much stronger acid than the sulfate anion is a base.

Unfortunately, this is kind of a problem that is hard to answer without guessing what the teacher had on mind [PLAIN]http://www.bpp.com.pl/IMG/grumpy_borek.png.