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.

 

[DeeNos]

As a scientist, it is important to consider all possible reactions and their extent when determining the pH of a solution. In this case, both reactions should be considered, as they contribute to the overall acidity of the solution. The first reaction, although of small extent, should not be ignored as it still contributes to the formation of H3O+ ions. However, the second reaction is expected to have a larger extent and should be given more weight in determining the overall pH of the solution. It is also important to consider the concentration of the sulfate ion, as it is a weak base and can also contribute to the overall pH. With all of these factors in mind, it is recommended to use a suitable equilibrium expression to calculate the pH of the solution.