`pK_(a)`of a weak acid is defined as :

Options

A. `log K_(a)`
B. `1/logK_(a)`
C. `"log"1/K_(a)`
D. `"-log" 1/K_(a)`

Explanation

To define the \( pK_a \) of a weak acid, we can follow these steps:

### Step 1: Understanding the Acid Dissociation Constant (\( K_a \))
The acid dissociation constant (\( K_a \)) is a measure of the strength of an acid in solution. It quantifies the extent to which an acid can donate protons (H⁺ ions) to the solution. The larger the \( K_a \), the stronger the acid, as it indicates a greater degree of dissociation.

### Step 2: Definition of \( pK_a \)
The \( pK_a \) is defined as the negative logarithm (base 10) of the acid dissociation constant (\( K_a \)):
\[ pK_a = -\log(K_a) \]

### Step 3: Relationship Between \( pK_a \) and Acid Strength
From the definition, we can infer that:
- A lower \( pK_a \) value indicates a stronger acid because it corresponds to a higher \( K_a \) value.
- Conversely, a higher \( pK_a \) value indicates a weaker acid.

### Step 4: Analyzing the Options
When presented with multiple-choice options regarding the definition of \( pK_a \):
1. Ensure that the formula includes the negative logarithm.
2. Check that the logarithm is applied correctly to the \( K_a \) value.
3. Verify that the relationship between \( pK_a \) and acid strength is correctly represented.

### Step 5: Conclusion
The correct definition of \( pK_a \) is:
\[ pK_a = -\log(K_a) \]
This indicates that as the strength of the acid increases (higher \( K_a \)), the \( pK_a \) decreases.