Number of Compounds Reacting with Phosphorus Pentachloride to Yield Phosphoryl Chloride

To determine the number of compounds that can react with phosphorus pentachloride ($\text{PCl}_5$) to yield phosphoryl chloride ($\text{POCl}_3$), let us examine the reaction of $\text{PCl}_5$ with each of the given species:

1. Reaction with $\text{O}_2$:
   Phosphorus in $\text{PCl}_5$ is already in its maximum oxidation state of $+5$. Hence, it cannot be oxidized further and does not react with $\text{O}_2$ to give $\text{POCl}_3$.
   $\text{PCl}_5 + \text{O}_2 \longrightarrow \text{No reaction}$

2. Reaction with $\text{CO}_2$:
   Carbon dioxide is thermodynamically very stable and an extremely weak oxygen-donor. It does not react with $\text{PCl}_5$.
   $\text{PCl}_5 + \text{CO}_2 \longrightarrow \text{No reaction}$

3. Reaction with $\text{SO}_2$:
   Sulfur dioxide reacts with $\text{PCl}_5$ via oxygen-chlorine exchange to produce phosphoryl chloride and thionyl chloride:
   $\text{PCl}_5 + \text{SO}_2 \longrightarrow \text{POCl}_3 + \text{SOCl}_2$

4. Reaction with $\text{H}_2\text{O}$:
   In limited water (or moist air), $\text{PCl}_5$ undergoes partial hydrolysis to produce phosphoryl chloride and hydrogen chloride:
   $\text{PCl}_5 + \text{H}_2\text{O} \longrightarrow \text{POCl}_3 + 2\text{HCl}$

5. Reaction with $\text{H}_2\text{SO}_4$:
   Sulfuric acid reacts with $\text{PCl}_5$ by replacing its hydroxyl groups with chlorine atoms, producing sulfuryl chloride (or chlorosulfuric acid) and $\text{POCl}_3$:
   $\text{H}_2\text{SO}_4 + 2\text{PCl}_5 \longrightarrow \text{SO}_2\text{Cl}_2 + 2\text{POCl}_3 + 2\text{HCl}$

6. Reaction with $\text{P}_4\text{O}_{10}$:
   Phosphorus pentoxide reacts readily with $\text{PCl}_5$ upon heating to form $\text{POCl}_3$:
   $6\text{PCl}_5 + \text{P}_4\text{O}_{10} \longrightarrow 10\text{POCl}_3$

Thus, the compounds that react with $\text{PCl}_5$ to give $\text{POCl}_3$ are $\text{SO}_2$, $\text{H}_2\text{O}$, $\text{H}_2\text{SO}_4$, and $\text{P}_4\text{O}_{10}$.

The total number of such compounds is 4.