The dissociation of a weak electrolyte obeys the law of mass action. It was found by

Options

A. Ostwald
B. Arrhenius
C. Berzelius
D. None of these

Explanation

To solve the question regarding which scientist established that the dissociation of a weak electrolyte obeys the law of mass action, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Law of Mass Action**:
- The law of mass action states that the rate of a chemical reaction is directly proportional to the product of the active masses (or concentrations) of the reactants. For a general reaction \( A + B \rightleftharpoons C + D \), the forward reaction rate can be expressed as:
\[ \text{Rate}_{\text{forward}} \propto [A][B] \]
and the backward reaction rate as:
\[ \text{Rate}_{\text{backward}} \propto [C][D] \]

2. **Equilibrium Condition**:
- At equilibrium, the rate of the forward reaction equals the rate of the backward reaction:
\[ \text{Rate}_{\text{forward}} = \text{Rate}_{\text{backward}} \]

3. **Application to Weak Electrolytes**:
- Weak electrolytes do not completely dissociate in solution. Ostwald's dilution law describes the equilibrium between the undissociated form of the electrolyte and its ions. This law can be derived using the law of mass action.

4. **Ostwald's Contribution**:
- Wilhelm Ostwald formulated the Ostwald's dilution law, which relates the degree of dissociation (α) of a weak electrolyte to its concentration (C) and the equilibrium constant (K). For a weak electrolyte of the form \( AB \), the equilibrium expression can be derived as:
\[ K = \frac{[B]^2 (1 - \alpha)}{C} \]
- In dilute solutions, where \( 1 - \alpha \approx 1 \), this simplifies to:
\[ K \approx \frac{[B]^2}{C} \]

5. **Conclusion**:
- The scientist who utilized the law of mass action to derive the dissociation of weak electrolytes is Wilhelm Ostwald. Therefore, the correct answer is option A: Ostwald.

### Final Answer:
The dissociation of a weak electrolyte obeys the law of mass action, and it was founded by **Ostwald** (Option A).