The number of `OH` units directly linked to bororn atom in `Na_(2)B_(4)O_(7).10 H_(2)O` is _____.

Explanation

To find the number of `OH` units directly linked to the boron atom in the compound `Na2B4O7·10H2O`, we can follow these steps:

### Step 1: Understand the Composition of the Compound
The compound `Na2B4O7·10H2O` is commonly known as borax. It can also be represented in a different way, which helps us identify the functional groups associated with boron.

### Step 2: Rewrite the Formula
The formula can be rewritten as `Na2B4O5(OH)4·8H2O`. This representation indicates that there are hydroxyl (OH) groups directly linked to the boron atoms.

### Step 3: Analyze the Tetra Nuclear Unit
The tetra nuclear unit in borax can be represented as `B4O5(OH)4^2-`. This means that there are 4 hydroxyl groups (OH) associated with the boron atoms in this structure.

### Step 4: Count the OH Units
From the rewritten formula and the tetra nuclear unit, we can see that there are 4 OH groups directly linked to the boron atoms.

### Conclusion
Thus, the number of `OH` units directly linked to the boron atom in `Na2B4O7·10H2O` is **4**.

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