The acid anhydride that will produce weak acid in water is

Analysis

The question asks us to identify which of the given acid anhydrides produces a weak acid when dissolved in water. An acid anhydride is a compound that reacts with water to form an acid.

Concept Explanation

• Acid Anhydride: These are typically non-metal oxides that react with water to form acidic solutions.
• Strong vs. Weak Acids: • Strong acids (like $H_2SO_4$ or $HNO_3$) dissociate completely in water. • Weak acids (like $H_2CO_3$ or $H_2SO_3$) dissociate only partially, meaning they exist in equilibrium with their ions.

Step-by-Step Solution

Let's look at the reaction of each option with water:

1. Option A ($SO_3$): $SO_3 + H_2O \rightarrow H_2SO_4$ (Sulfuric acid). This is a strong acid.
2. Option B ($NO_2$): $2NO_2 + H_2O \rightarrow HNO_3 + HNO_2$ (Nitric acid and Nitrous acid). Nitric acid is a strong acid.
3. Option C ($SO_2$): $SO_2 + H_2O \rightleftharpoons H_2SO_3$ (Sulfurous acid). While this is an acid, it is relatively weak compared to sulfuric acid, but let's compare it with $CO_2$.
4. Option D ($CO_2$): $CO_2 + H_2O \rightleftharpoons H_2CO_3$ (Carbonic acid). Carbonic acid is a classic example of a weak acid used in carbonated drinks. It is very unstable and dissociates only slightly in water.

Comparing the options, $CO_2$ produces carbonic acid, which is significantly weaker than the mineral acids produced by the other oxides listed. In the context of standard chemistry examinations, $CO_2$ is the standard answer for a weak acid anhydride.

Conclusion

Carbonic acid ($H_2CO_3$) is a very weak acid compared to the others listed, which form strong mineral acids.

Exam Tip

Remember the "Strong Acid" rule: Most oxides of non-metals in high oxidation states (like $S$ in $+6$ or $N$ in $+5$) form strong acids. Carbon, being in a lower oxidation state ($+4$) and forming a weak, unstable acid, is your go-to for "weak acid" questions.