It required 0.7g of hydrocarbon (A) to react completely with 2.0g of `Br_(2)`. On treatment of 'A' with HBr it gave monobromo alkane (B). The same compound (B) was obtained when (A) was treated with HBr in presence of peroxide. Calculate the molecular weight of 'B'.

Answer

To calculate the molecular weight of compound B, we will follow these steps:

### Step 1: Calculate the moles of Br₂
We know that the molecular weight of Br₂ (bromine) is 160 g/mol (since Br has a molecular weight of 80 g/mol).

Using the formula:
\[ \text{Moles of Br₂} = \frac{\text{Weight of Br₂}}{\text{Molecular weight of Br₂}} \]
Substituting the values:
\[ \text{Moles of Br₂} = \frac{2.0 \, \text{g}}{160 \, \text{g/mol}} = 0.0125 \, \text{mol} \]

### Step 2: Determine the moles of hydrocarbon A
Since 1 mole of unsaturated hydrocarbon reacts with 1 mole of Br₂, the moles of hydrocarbon A will also be 0.0125 mol.

### Step 3: Calculate the molecular weight of hydrocarbon A
Using the formula:
\[ \text{Molecular weight of A} = \frac{\text{Weight of A}}{\text{Moles of A}} \]
Substituting the values:
\[ \text{Molecular weight of A} = \frac{0.7 \, \text{g}}{0.0125 \, \text{mol}} = 56 \, \text{g/mol} \]

### Step 4: Determine the molecular formula of hydrocarbon A
The molecular weight of hydrocarbon A is 56 g/mol. To find the number of carbon (C) and hydrogen (H) atoms, we can use the following relationships:
- Each carbon atom contributes 12 g/mol.
- Each hydrogen atom contributes 1 g/mol.

Let \( x \) be the number of carbon atoms and \( y \) be the number of hydrogen atoms:
\[ 12x + y = 56 \]

Since A is an alkene (unsaturated hydrocarbon), we can use the general formula for alkenes, which is \( C_nH_{2n} \).

Assuming \( n = 4 \) (as we will find out later):
\[ y = 2n = 2 \times 4 = 8 \]

Thus, the molecular formula of A is \( C_4H_8 \).

### Step 5: Determine the molecular formula of compound B
When hydrocarbon A reacts with HBr, it forms a monobromo alkane B. Therefore, the molecular formula of B will be:
\[ B = C_4H_7Br \]

### Step 6: Calculate the molecular weight of compound B
To find the molecular weight of B:
\[ \text{Molecular weight of B} = (4 \times 12) + (7 \times 1) + (1 \times 80) \]
Calculating:
\[ \text{Molecular weight of B} = 48 + 7 + 80 = 135 \, \text{g/mol} \]

### Final Answer
The molecular weight of compound B is **135 g/mol**.