1.25g of a solid dibasic acid is completely neutralised by 25mL of 0.25 molar `Ba(OH_(2))` solution. Molecular mass of the acid is:

Options

A. 100
B. 150
C. 120
D. 200

Answer

To find the molecular mass of the dibasic acid that is completely neutralized by barium hydroxide, we can follow these steps:

### Step 1: Determine the number of equivalents of barium hydroxide (Ba(OH)₂)
The number of equivalents can be calculated using the formula:
\[ \text{Number of equivalents} = \text{Molarity} \times \text{Volume in liters} \times n \]
For barium hydroxide, \( n = 2 \) (since it can donate 2 hydroxide ions).

Given:
- Molarity of Ba(OH)₂ = 0.25 M
- Volume = 25 mL = 0.025 L

Calculating the number of equivalents:
\[ \text{Number of equivalents of Ba(OH)₂} = 0.25 \, \text{mol/L} \times 0.025 \, \text{L} \times 2 = 0.0125 \, \text{equivalents} \]

### Step 2: Determine the number of equivalents of the dibasic acid (H₂A)
Since the dibasic acid is completely neutralized by barium hydroxide, the number of equivalents of the acid will be equal to the number of equivalents of barium hydroxide:
\[ \text{Number of equivalents of H₂A} = 0.0125 \, \text{equivalents} \]

### Step 3: Calculate the number of moles of the dibasic acid (H₂A)
The number of equivalents is also related to the number of moles by the formula:
\[ \text{Number of equivalents} = \text{Number of moles} \times n \]
For the dibasic acid, \( n = 2 \).

Rearranging the formula to find the number of moles:
\[ \text{Number of moles of H₂A} = \frac{\text{Number of equivalents}}{n} = \frac{0.0125}{2} = 0.00625 \, \text{moles} \]

### Step 4: Calculate the molecular mass of the dibasic acid
Using the formula:
\[ \text{Number of moles} = \frac{\text{Weight}}{\text{Molecular mass}} \]
Rearranging to find the molecular mass:
\[ \text{Molecular mass} = \frac{\text{Weight}}{\text{Number of moles}} \]
Given that the weight of the acid is 1.25 g:
\[ \text{Molecular mass} = \frac{1.25 \, \text{g}}{0.00625 \, \text{moles}} = 200 \, \text{g/mol} \]

### Conclusion
The molecular mass of the dibasic acid is **200 g/mol**.

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