What is meant by 'acidity constant', `K_(a)`? How is it expressed?

Explanation

### Step-by-Step Solution:

1. **Definition of Acidity Constant (K_a)**:
The acidity constant, denoted as \( K_a \), is a quantitative measure of the strength of an acid in solution. It indicates how well an acid donates protons (H⁺ ions) in an aqueous solution.

2. **Dissociation of an Acid**:
When an acid (for example, a carboxylic acid represented as \( RCOOH \)) is dissolved in water, it dissociates into two species: the hydronium ion (\( H_3O^+ \)) and its conjugate base (\( RCOO^- \)). The dissociation can be represented as:
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