Which is not true about borax ?

Options

A. It is a useful primary standard for titrating against acids
B. One mole of borax can be used as a buffer
C. Aqueous solution of borax can not be used as buffer
D. It is made up of two triangular `BO_(3)` units and two tetrahedral `BO_(4)` units

Answer

To determine which statement about borax is not true, we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Understanding Borax**:
- Borax is a white crystalline powder, chemically known as sodium tetraborate. Its formula is Na2B4O7·10H2O (decahydrate form).
- It is commonly used in various applications, including as a cleaning agent and in glassmaking.

2. **Analyzing Each Statement**:
- **Option A**: "It is a useful primary standard for titrating against acids."
- This statement is **true**. Borax can be used as a primary standard in acid-base titrations because it is stable and can be accurately weighed.

- **Option B**: "One mole of borax can be used as a buffer."
- This statement is **true**. Borax can act as a buffer solution because it can hydrolyze in water to form boric acid and its conjugate base, thus maintaining pH.

- **Option C**: "Aqueous solution of borax cannot be used as a buffer."
- This statement is **not true**. An aqueous solution of borax can indeed act as a buffer when it hydrolyzes, providing a weak acid and its conjugate base.

- **Option D**: "It is made up of two triangular B(OH)3 units and two tetrahedral B(OH)4 units."
- This statement is **true**. The structure of borax includes these units, confirming its composition.

3. **Conclusion**:
- The statement that is not true about borax is **Option C**: "Aqueous solution of borax cannot be used as a buffer."

### Final Answer:
**Option C** is the statement that is not true about borax.

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