`underset(("Hydrated salt"))((A)) underset("heated")overset("Storngly")tounderset("glassy bead")underset("transparent")ubrace(B+C)`
`H_(3)BO_(3) underset("heated")overset("Strongly")toC+D`
Find the number of tetrahedral and trigonal planar units in structure of A -

Options

A. 2,1
B. 2,2
C. 2,4
D. 5,2

Answer

To solve the problem, we need to analyze the reactions and structures involved in the transformation of the hydrated salt (A) and the orthoboric acid (H₃BO₃). Let's break it down step by step.

### Step 1: Identify the Compounds
- **Compound A**: The hydrated salt is identified as borax, which has the formula Na₂B₄O₇·10H₂O.
- **Compound C**: The product formed upon heating is B₂O₃ (boric anhydride).
- **Compound B**: The transparent glassy bead formed during the heating process.

### Step 2: Write the Reaction for Orthoboric Acid
The reaction for orthoboric acid (H₃BO₃) upon strong heating can be written as:
\[ \text{H}_3\text{BO}_3 \xrightarrow{\text{heat}} \text{B}_2\text{O}_3 + \text{H}_2\text{O} \]
This shows that heating H₃BO₃ produces boric anhydride (B₂O₃) and water.

### Step 3: Write the Reaction for Hydrated Salt (A)
The reaction for borax (A) upon strong heating can be written as:
\[ \text{Na}_2\text{B}_4\text{O}_7 \cdot 10\text{H}_2\text{O} \xrightarrow{\text{heat}} \text{B}_2\text{O}_3 + \text{Na}_2\text{B}_2\text{O}_5 + \text{H}_2\text{O} \]
This indicates that borax decomposes to form boric anhydride (C) and a glassy bead (B).

### Step 4: Analyze the Structure of Borax (A)
The structure of borax (Na₂B₄O₇·10H₂O) consists of:
- **Trigonal Planar Units**: These are formed by the boron atoms in the structure, which are coordinated to three oxygen atoms. In borax, there are 2 trigonal planar units.
- **Tetrahedral Units**: These are formed by the boron atoms that are coordinated to four oxygen atoms. In borax, there are also 2 tetrahedral units.

### Step 5: Count the Units
From the structure of borax, we can conclude:
- Number of **Trigonal Planar Units**: 2
- Number of **Tetrahedral Units**: 2

### Final Answer
Thus, the number of tetrahedral and trigonal planar units in the structure of A (borax) is:
- **Trigonal Planar Units**: 2
- **Tetrahedral Units**: 2

### Summary
The final answer is:
- **2 trigonal planar units and 2 tetrahedral units.**

---