The increasing order of acidic strength for the aqua complexes [ V(H ₂ O) ₆]²⁺ , [ Fe(H ₂ O) ₆]²⁺ , [ Fe(H ₂ O) ₆]³⁺ , and [ Co(H ₂ O) ₆]³⁺ is:

Step-by-step solution

The acidic strength of an aqua complex is determined by the polarizing power of the central metal ion, which is directly proportional to its charge and inversely proportional to its ionic radius (charge-to-size ratio). Higher oxidation states lead to a greater charge density. Therefore, the +3 metal ions ( Fe ³⁺ , Co ³⁺ ) have a significantly higher polarizing power than the +2 metal ions ( V ²⁺ , Fe ²⁺ ). This makes the +3 aqua complexes more acidic than the +2 aqua complexes. For the +2 ions, as we move from left