The element which can combine with oxygen to form an acid anhydride of the form XO$_2$ is

Analysis

The question asks us to identify which of the given elements forms an acid anhydride with the general formula $XO_2$. An acid anhydride is an oxide that reacts with water to form an acid. We need to check which of the options forms a stable dioxide ($XO_2$) that acts as an acid anhydride.

Concept Explanation

• Acid Anhydride: These are typically non-metal oxides. When they dissolve in water, they produce an acid.
• The Formula $XO_2$: We are looking for an element $X$ that forms a stable oxide where the ratio of oxygen to the element is 2:1.
• Evaluating the options: • Phosphorus (P): Commonly forms $P_4O_{10}$ or $P_4O_6$. It does not typically exist as a simple $PO_2$ molecule. • Chlorine (Cl): Forms oxides like $Cl_2O$, $ClO_2$, and $Cl_2O_7$. While $ClO_2$ exists, it is a radical and not the most common representative of acid anhydrides in this context. • Fluorine (F): Fluorine is highly electronegative and does not form stable acid anhydrides with oxygen; it forms oxygen fluorides (like $OF_2$) where oxygen is the more electronegative atom. • Sulphur (S): Sulphur reacts with oxygen to form Sulphur(IV) oxide ($SO_2$). $SO_2$ is a classic acid anhydride because it reacts with water to form sulphurous acid ($H_2SO_3$): $$SO_2(g) + H_2O(l) \rightarrow H_2SO_3(aq)$$

Step-by-Step Solution

1. Identify the oxide: We are looking for an oxide of the form $XO_2$.
2. Test Sulphur: Sulphur burns in oxygen to produce $SO_2$.
3. Verify the property: $SO_2$ is a non-metal oxide. Non-metal oxides are acidic in nature.
4. Check the reaction with water: $SO_2 + H_2O \rightarrow H_2SO_3$. Since it produces an acid, it is an acid anhydride.
5. Compare with others: While $ClO_2$ exists, $SO_2$ is the standard textbook example for an acid anhydride of the form $XO_2$ in Nigerian secondary school chemistry curricula.

Conclusion

Sulphur is the element that forms the stable acid anhydride $SO_2$.

Exam Tip

Remember the "Non-metal Oxide Rule": Most non-metal oxides are acidic (acid anhydrides), while most metal oxides are basic. When you see a question about acid anhydrides, look for the non-metal that forms a stable oxide with oxygen!