Cl, Br, I follows Dobereiner's triads, the atomic masses of Cl and I are 35.5 and 127 respectively. The atomic mass of Br is :

Explanation

To find the atomic mass of bromine (Br) using Dobereiner's triads, we can follow these steps:

### Step 1: Understand Dobereiner's Triads
Dobereiner's triads suggest that when three elements are grouped together, the atomic mass of the middle element is approximately the average of the atomic masses of the other two elements.

### Step 2: Identify Given Values
From the question, we have:
- Atomic mass of Chlorine (Cl) = 35.5
- Atomic mass of Iodine (I) = 127

### Step 3: Apply the Triad Concept
According to Dobereiner's triads, the atomic mass of Bromine (Br) can be calculated as follows:
\[ \text{Atomic mass of Br} = \frac{\text{Atomic mass of Cl} + \text{Atomic mass of I}}{2} \]

### Step 4: Substitute the Values
Now, substituting the given values into the equation:
\[ \text{Atomic mass of Br} = \frac{35.5 + 127}{2} \]

### Step 5: Calculate the Sum
First, calculate the sum of the atomic masses:
\[ 35.5 + 127 = 162.5 \]

### Step 6: Divide by 2
Now, divide the sum by 2 to find the atomic mass of Br:
\[ \text{Atomic mass of Br} = \frac{162.5}{2} = 81.25 \]

### Conclusion
Thus, the atomic mass of Bromine (Br) is 81.25 g/mol.

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