Metal Aqua Ion Acidity Comparison

The acidity of metal aqua ions, such as $[Fe(H_2O)_6]^{3+}$ and $[Fe(H_2O)_6]^{2+}$, depends on the charge density of the central metal ion. A higher charge on the metal ion increases its attraction for the electrons in the O-H bonds of the coordinated water molecules, making it easier to release a proton ($H^+$).

• $[Fe(H_2O)_6]^{3+}$ contains $Fe^{3+}$ (charge +3).
• $[Fe(H_2O)_6]^{2+}$ contains $Fe^{2+}$ (charge +2).
• $Fe^{3+}$ has a higher charge density than $Fe^{2+}$.
• Consequently, $[Fe(H_2O)_6]^{3+}$ is a stronger acid than $[Fe(H_2O)_6]^{2+}$.
• Stronger acids have lower $pK_a$ values. Therefore, $pK_a([Fe(H_2O)_6]^{3+}) < pK_a([Fe(H_2O)_6]^{2+})$.

Acidic Oxide Acidity Comparison

The acidity of oxides like $N_2O_5$ and $V_2O_5$ depends on the nature of the central atom and its oxidation state. Generally, non-metal oxides are more acidic than metal oxides, especially when comparing elements in similar oxidation states.

• $N_2O_5$ is the oxide of nitrogen (a non-metal) in the +5 oxidation state. It is a potent acidic oxide, forming nitric acid ($HNO_3$) upon reaction with water.
• $V_2O_5$ is the oxide of vanadium (a transition metal) in the +5 oxidation state. It acts as an acidic oxide, forming vanadic acid, but is considerably weaker than nitric acid.
• Non-metal oxides are typically more acidic than metal oxides. Thus, $N_2O_5$ is a stronger acid than $V_2O_5$.
• Therefore, $pK_a(N_2O_5) < pK_a(V_2O_5)$.

Overall $pK_a$ Comparison

Combining the findings for both pairs:

• For metal aqua ions: $pK_a([Fe(H_2O)_6]^{3+}) < pK_a([Fe(H_2O)_6]^{2+})$.
• For acidic oxides: $pK_a(N_2O_5) < pK_a(V_2O_5)$.

The correct comparison statement is $[Fe(H_2O)_6]^{3+} < [Fe(H_2O)_6]^{2+}$ and $N_2O_5 < V_2O_5$.