The number of moles of an acid or base added to one litre of the buffer solution so as to change its pH by one unit is called............of the buffer.

Answer

To answer the question, we need to identify the term that describes the number of moles of an acid or base that can be added to one liter of a buffer solution to change its pH by one unit.

### Step-by-step Solution:

1. **Understanding Buffer Solutions**:
- A buffer solution is designed to resist changes in pH when small amounts of acid or base are added. It typically consists of a weak acid and its conjugate base (or a weak base and its conjugate acid).

2. **Definition of Buffer Capacity**:
- The term we are looking for is related to the ability of the buffer to maintain its pH. This ability is quantified as the buffer capacity, which is defined as the amount of acid or base that can be added to a buffer solution before a significant change in pH occurs.

3. **Identifying the Term**:
- The specific term that describes the number of moles of acid or base needed to change the pH of one liter of buffer solution by one unit is known as the **buffer capacity** or **buffer index**.

4. **Conclusion**:
- Therefore, the answer to the question is: "The number of moles of an acid or base added to one liter of the buffer solution so as to change its pH by one unit is called the **buffer capacity** of the buffer."