Which of the following statements are INCORRECT? A. All the transition metals except scandium form MO oxides which are ionic. B. The highest oxidation number corresponding to the group number in transition metal oxides is attained in $\mathrm{Sc}{}_2\mathrm{O}{}_3$ to $\mathrm{Mn}{}_2\mathrm{O}{}_7$. C. Basic character increases from $\mathrm{V}{}_2\mathrm{O}{}_3$ to $\mathrm{V}{}_2\mathrm{O}{}_4$ to $\mathrm{V}{}_2\mathrm{O}{}_5$. D. $\mathrm{V}{}_2\mathrm{O}{}_4$ dissolves in acids to give $\mathrm{VO}{}_4{}^{3-}$ salts. E. $\mathrm{CrO}$ is basic but $\mathrm{Cr}{}_2\mathrm{O}{}_3$ is amphoteric. Choose the correct answer from the options given below:

Solution & Explanation

\textbf{Answer:} (c) C and D only \textbf{Solution:} For transition-metal oxides, basic character \textbf{decreases} (and acidic character increases) as the oxidation state of the metal rises. So going from $\mathrm{V}{}_2\mathrm{O}{}_3$ (+3) to $\mathrm{V}{}_2\mathrm{O}{}_4$ (+4) to $\mathrm{V}{}_2\mathrm{O}{}_5$ (+5), the basic character decreases, not increases. Hence Statement C is incorrect. When $\mathrm{V}{}_2\mathrm{O}{}_4$ dissolves in acids it gives the vanadyl ion $\mathrm{VO}{}^{2+}$ (i.e. $\mathrm{VO}{}^{2+}$ salts), not the vanadate $\mathrm{VO}{}_4{}^{3-}$ (which corresponds to V in the +5 state). Hence Statement D is incorrect. Statements A, B and E are correct, so the INCORRECT pair is C and D.