For a first order reaction, `A rarr B`, the rate `= k xx………….` .

Answer

To solve the question regarding the rate of a first-order reaction \( A \rightarrow B \), we will follow these steps:

### Step-by-Step Solution:

1. **Identify the Reaction Type**:
- The reaction given is a first-order reaction where \( A \) is the reactant and \( B \) is the product.

2. **Write the Rate Law**:
- For a first-order reaction, the rate of the reaction is directly proportional to the concentration of the reactant. This can be expressed mathematically as:
\[ \text{Rate} \propto [A] \]
- Here, \([A]\) represents the concentration of reactant \( A \).

3. **Convert Proportionality to an Equation**:
- To convert the proportionality into an equation, we introduce a rate constant \( k \):
\[ \text{Rate} = k \cdot [A]^1 \]
- Since the reaction is first-order, the exponent of \([A]\) is 1.

4. **Final Rate Expression**:
- Therefore, the final expression for the rate of the reaction is:
\[ \text{Rate} = k \cdot [A] \]

### Final Answer:
The rate of the first-order reaction \( A \rightarrow B \) is given by:
\[ \text{Rate} = k \cdot [A] \]