In the formation of `SO_(3)` by contact process, the conditions used are

Options

A. Catalyst, adding excess of `O_(2)` to `SO_(2) ` at `450^(@)`C and 2 atm
B. Catalyst, adding excess of `SO_(2)` to `H_(2)SO_(4)` at `100^(@)C` and 20 atm
C. Catalyst, adding excess of `SO_(2)` to traces of `O_(2)` at `20^(@)C` and 1 atm
D. Catalyst, adding excess of `H_(2)SO_(4)` to `O_(2)` at `30^(@)C` and 30 atm

Answer

To determine the conditions used in the formation of SO3 by the contact process, we need to analyze the reaction and the factors that influence it. The reaction for the formation of sulfur trioxide (SO3) is given by:

\[ 2SO_2 + O_2 \rightleftharpoons 2SO_3 \]

### Step-by-Step Solution:

1. **Identify the Reaction**:
- The contact process involves the reaction of sulfur dioxide (SO2) with oxygen (O2) to form sulfur trioxide (SO3).

2. **Understand the Role of Temperature**:
- The reaction is exothermic, meaning it releases heat. According to Le Chatelier's principle, increasing the temperature will shift the equilibrium to favor the reactants (SO2 and O2). Therefore, a moderate temperature is preferred to favor the formation of SO3.

3. **Consider the Pressure**:
- The reaction involves a decrease in the number of moles of gas (3 moles of reactants to 2 moles of products). Increasing the pressure will favor the side with fewer moles of gas, which is the product side (SO3). Thus, high pressure is advantageous for the formation of SO3.

4. **Use of Catalyst**:
- A catalyst is used to speed up the reaction without being consumed in the process. In the contact process, vanadium pentoxide (V2O5) is commonly used as a catalyst. The presence of a catalyst helps achieve equilibrium faster but does not affect the position of equilibrium.

5. **Optimal Conditions**:
- The optimal conditions for the contact process typically involve:
- A temperature of around 450°C
- A pressure of about 2 atmospheres
- The use of a catalyst (V2O5)

6. **Conclusion**:
- The correct conditions for the formation of SO3 in the contact process are: using a catalyst, maintaining a high pressure, and a moderate temperature.

### Final Answer:
The conditions used in the formation of SO3 by the contact process include:
- Use of a catalyst (e.g., V2O5)
- High pressure (around 2 atm)
- Moderate temperature (around 450°C)