To determine which compounds react with PCl₅ to form POCl₃, we need to understand the chemical behavior of phosphorus pentachloride (PCl₅). PCl₅ is a reactive compound that can act as a chlorinating agent and can undergo reactions where it donates chlorine atoms or accepts oxygen atoms to form phosphoryl chloride (POCl₃). The general reaction for the formation of POCl₃ from PCl₅ involves the replacement of chlorine with oxygen, often requiring a source of oxygen or a compound that can provide oxygen.

The reaction is typically: $\mathrm{PCl}5+O-\text{source}\to \mathrm{POCl}3+\text{other products}$

Now, let's evaluate each compound:

Step 1: O₂ (Oxygen) - Oxygen does not directly react with PCl₅ to form POCl₃ under normal conditions. PCl₅ is stable in oxygen and does not undergo oxidation to POCl₃ easily. So, no reaction.

Step 2: CO₂ (Carbon dioxide) - CO₂ does not react with PCl₅. It is inert and does not provide oxygen in a form that can chlorinate or react with PCl₅. So, no reaction.

Step 3: SO₂ (Sulfur dioxide) - SO₂ reacts with PCl₅ to form POCl₃ and thionyl chloride (SOCl₂). The reaction is: $\mathrm{PCl}5+\mathrm{SO}2\to \mathrm{POCl}3+\mathrm{SOCl}2$. Yes, it reacts.

Step 4: H₂O (Water) - Water hydrolyzes PCl₅ to phosphoric acid and HCl, not directly to POCl₃. The reaction is: $\mathrm{PCl}5+4H2O\to H3\mathrm{PO}4+5\mathrm{HCl}$. POCl₃ is an intermediate but is not the stable product; it further hydrolyzes. So, it does not give POCl₃ as a final product in a simple reaction. Typically, it is not considered to directly give POCl₃ with H₂O.

Step 5: H₂SO₄ (Sulfuric acid) - Concentrated H₂SO₄ reacts with PCl₅ to form POCl₃, HCl, and SO₂Cl₂ (or similar products). The reaction is: $2\mathrm{PCl}5+H2\mathrm{SO}4\to 2\mathrm{POCl}3+2\mathrm{HCl}+\mathrm{SO}2\mathrm{Cl}2$. Yes, it reacts to give POCl₃.

Step 6: P₄O₁₀ (Phosphorus pentoxide) - P₄O₁₀ reacts with PCl₅ to form POCl₃. The reaction is: $P4O10+6\mathrm{PCl}5\to 10\mathrm{POCl}3$. Yes, it reacts.

From the analysis, SO₂, H₂SO₄, and P₄O₁₀ react with PCl₅ to give POCl₃. O₂, CO₂, and H₂O do not.

Final Answer: The number of compounds is 3 (SO₂, H₂SO₄, P₄O₁₀).

Related Topics and Formulae

Reactivity of PCl₅: Phosphorus pentachloride is used in chlorination reactions and can convert carboxylic acids to acid chlorides, alcohols to alkyl chlorides, and with oxygen-containing compounds, it can form POCl₃.

Key Reactions:

• With SO₂: $\mathrm{PCl}5+\mathrm{SO}2\to \mathrm{POCl}3+\mathrm{SOCl}2$
• With H₂SO₄: $2\mathrm{PCl}5+H2\mathrm{SO}4\to 2\mathrm{POCl}3+2\mathrm{HCl}+\mathrm{SO}2\mathrm{Cl}2$
• With P₄O₁₀: $P4O10+6\mathrm{PCl}5\to 10\mathrm{POCl}3$

Theory: POCl₃ (phosphoryl chloride) is an important compound in organic and inorganic chemistry, used as a chlorinating agent and in the manufacture of other phosphorus compounds. The reactions typically involve the transfer of oxygen from the reactant to phosphorus, displacing chlorine.