Sulphur trioxide can be obtained by which of the following reactions:

Options

A. `CaSO_(4) + Coverset(Delta)rarr`
B. `Fe_(2)(SO_(4))_(3) overset(Delta)rarr`
C. `S+H_(2)SO_(4) overset(Delta)rarr`
D. `H_(2)SO_(4) + PCl_(5) overset(Delta)rarr`

Explanation

To determine which reactions can produce sulfur trioxide (SO3), we will analyze each given reaction step by step.

### Step 1: Analyze Reaction with Calcium Sulfate and Carbon
The first reaction involves calcium sulfate (CaSO4) reacting with carbon (C) in the presence of heat.

**Reaction:**
\[ \text{CaSO}_4 + \text{C} \xrightarrow{\text{heat}} \text{CaO} + \text{SO}_2 + \text{CO} \]

**Products:**
- Calcium oxide (CaO)
- Sulfur dioxide (SO2)
- Carbon monoxide (CO)

**Conclusion:**
This reaction produces sulfur dioxide (SO2), not sulfur trioxide (SO3).

### Step 2: Analyze Reaction with Ferric Sulfate
The second reaction involves the decomposition of ferric sulfate (Fe2(SO4)3) upon heating.

**Reaction:**
\[ \text{Fe}_2(\text{SO}_4)_3 \xrightarrow{\text{heat}} \text{Fe}_2\text{O}_3 + \text{SO}_3 \]

**Products:**
- Ferric oxide (Fe2O3)
- Sulfur trioxide (SO3)

**Conclusion:**
This reaction produces sulfur trioxide (SO3). Thus, this is a valid reaction for producing SO3.

### Step 3: Analyze Reaction with Sulfur and Sulfuric Acid
The third reaction involves sulfur (S) reacting with sulfuric acid (H2SO4).

**Reaction:**
\[ \text{S} + \text{H}_2\text{SO}_4 \xrightarrow{\text{heat}} \text{SO}_2 + \text{H}_2\text{O} \]

**Products:**
- Sulfur dioxide (SO2)
- Water (H2O)

**Conclusion:**
This reaction produces sulfur dioxide (SO2), not sulfur trioxide (SO3).

### Step 4: Analyze Reaction with Sulfuric Acid and Phosphorus Pentachloride
The fourth reaction involves sulfuric acid (H2SO4) reacting with phosphorus pentachloride (PCl5).

**Reaction:**
\[ \text{H}_2\text{SO}_4 + \text{PCl}_5 \xrightarrow{\text{heat}} \text{SO}_3\text{HCl} \]

**Products:**
- Chlorosulfonic acid (SO3HCl)

**Conclusion:**
This reaction produces chlorosulfonic acid (SO3HCl), not sulfur trioxide (SO3).

### Final Conclusion
After analyzing all the reactions, we find that only the decomposition of ferric sulfate (Fe2(SO4)3) produces sulfur trioxide (SO3). Therefore, the correct answer is:

**Option B: Ferric sulfate decomposing produces sulfur trioxide.**

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