Question

Find the correct matches.

• A. \( A = R,\; B = S,\; C = P,\; D = Q \)
• B. \( A = S,\; B = R,\; C = P,\; D = Q \)
• C. \( A = R,\; B = Q,\; C = S,\; D = P \)
• D. \( A = S,\; B = R,\; C = Q,\; D = P \)

Hint:

For transition metal oxides, lower oxidation states are generally basic while higher oxidation states are acidic. Also, chromate and dichromate ions interconvert depending on pH.

Answer 1

The Correct Option is D

Step 1: Analyze \( Cr_2O_7^{2-} \).
Dichromate ion \( Cr_2O_7^{2-} \) is interconvertible with chromate ion \( CrO_4^{2-} \) depending on the pH of the medium.
In acidic medium, dichromate ion is favoured.
In basic medium, chromate ion is favoured.
\[ Cr_2O_7^{2-} + H_2O \rightleftharpoons 2CrO_4^{2-} + 2H^+ \]
So:
\[ A = S \]

Step 2: Analyze \( FeCr_2O_4 \).
\( FeCr_2O_4 \) is chromite ore.
When chromite ore is fused with \( Na_2CO_3 \) in air, it forms yellow sodium chromate.
\[ 4FeCr_2O_4 + 8Na_2CO_3 + 7O_2 \rightarrow 8Na_2CrO_4 + 2Fe_2O_3 + 8CO_2 \]
So:
\[ B = R \]

Step 3: Analyze \( CrO_3 \).
\( CrO_3 \) is chromium trioxide.
It is an acidic oxide because chromium is present in a high oxidation state \( +6 \).
Acidic character generally increases with increase in oxidation state of the metal.
So:
\[ C = Q \]

Step 4: Analyze \( CrO \).
In \( CrO \), chromium is present in \( +2 \) oxidation state.
Metal oxides in lower oxidation states are generally basic in nature.
Therefore, \( CrO \) is predominantly basic.
So:
\[ D = P \]

Step 5: Write the complete matching.
From the above analysis:
\[ A = S,\quad B = R,\quad C = Q,\quad D = P \]

Step 6: Match with the given options.
The matching \( A = S,\; B = R,\; C = Q,\; D = P \) corresponds to option (D).

Step 7: Conclusion.
Thus, the correct matches are:
\[ \boxed{A = S,\; B = R,\; C = Q,\; D = P} \]