0.45 gm of an acid with molecular mass 90 g/mole is neutralized by 20 ml of 0.5 N caustic potash. The basicity of the acid is :

Options

A. 1
B. 2
C. 3
D. 4

Explanation

To find the basicity of the acid, we will follow these steps:

### Step 1: Calculate the number of equivalents of the base (caustic potash, NaOH).
The formula for calculating the number of equivalents is:

\[ \text{Number of equivalents} = \text{Normality} \times \text{Volume in liters} \]

Given:
- Normality (N) = 0.5 N
- Volume (V) = 20 ml = 20/1000 L = 0.02 L

Substituting the values:

\[ \text{Number of equivalents of NaOH} = 0.5 \, \text{N} \times 0.02 \, \text{L} = 0.01 \, \text{equivalents} \]

### Step 2: Relate the equivalents of acid to the equivalents of base.
In a neutralization reaction, the number of equivalents of acid is equal to the number of equivalents of base. Therefore:

\[ \text{Number of equivalents of acid} = 0.01 \, \text{equivalents} \]

### Step 3: Calculate the number of moles of the acid.
The number of equivalents of an acid can be calculated using the formula:

\[ \text{Number of equivalents} = \frac{\text{mass of acid}}{\text{molecular mass}} \times \text{basicity} \]

Given:
- Mass of acid = 0.45 g
- Molecular mass of acid = 90 g/mol

Let the basicity be \( n \). Thus, we can write:

\[ 0.01 = \frac{0.45}{90} \times n \]

### Step 4: Solve for the basicity (n).
First, calculate the number of moles of the acid:

\[ \frac{0.45}{90} = 0.005 \, \text{moles} \]

Now substitute this back into the equation:

\[ 0.01 = 0.005 \times n \]

To find \( n \):

\[ n = \frac{0.01}{0.005} = 2 \]

### Conclusion:
The basicity of the acid is 2, which means it is a dibasic acid.

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