In which of these processes platinum is used as a catalyst ? 

Options

A. Oxidation of ammonia to form `HNO_(3)`
B. Hardening of oils
C. Production of synthetic rubber
D. Synthesis of methanol

Answer

To determine in which process platinum is used as a catalyst, we can analyze the given options and focus on the specific reactions where platinum plays a catalytic role.

### Step-by-Step Solution:

1. **Identify the Processes**: We need to look at the different processes mentioned in the options and see if any of them are known to utilize platinum as a catalyst.

2. **Understand the Oswald Process**: The Oswald process is a well-known method for the preparation of nitric acid. It involves the oxidation of ammonia (NH₃) in the presence of oxygen (O₂) at high temperatures (around 750°C) using platinum as a catalyst.

3. **Write the Reaction**: The first step in the Oswald process can be represented as:
\[ 4 NH₃(g) + 5 O₂(g) \xrightarrow{Pt, 750°C} 4 NO(g) + 6 H₂O(g) \]
Here, platinum (Pt) facilitates the reaction without being consumed.

4. **Subsequent Reactions**: The nitrogen monoxide (NO) produced can further react with oxygen to form nitrogen dioxide (NO₂):
\[ 2 NO(g) + O₂(g) \rightarrow 2 NO₂(g) \]

5. **Final Product Formation**: Finally, nitrogen dioxide can react with water to produce nitric acid (HNO₃):
\[ 3 NO₂(g) + H₂O(l) \rightarrow 2 HNO₃(aq) + NO(g) \]

6. **Conclusion**: Since platinum is used as a catalyst in the Oswald process for the preparation of nitric acid, we can conclude that this is the correct answer.

### Final Answer:
Platinum is used as a catalyst in the Oswald process for the preparation of nitric acid.

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