Active mass is defined as

Options

A. Number of g equivalent per unit volume
B. Number of g mol per L
C. Amount of substance in g per unit volume
D. Number of g mol in `100 L`

Explanation

### Step-by-Step Solution:

1. **Understanding Active Mass**:
- Active mass is not a measure of mass in the traditional sense but rather a measure of concentration. It is important in the context of chemical equilibrium and reaction rates.

2. **Definition of Active Mass**:
- Active mass is defined as the concentration of a substance that affects the rate of a reaction. It is the amount of substance present in a given volume.

3. **Relation to Molarity**:
- The active mass can be expressed in terms of molarity, which is defined as the number of moles of solute per liter of solution. Therefore, active mass can be represented as:
\[ \text{Active Mass} = \frac{\text{Number of moles}}{\text{Volume in liters}} \]

4. **Calculating Moles**:
- The number of moles can be calculated using the formula:
\[ \text{Number of moles} = \frac{\text{Weight of substance (g)}}{\text{Molar mass (g/mol)}} \]
- Thus, active mass can also be expressed as:
\[ \text{Active Mass} = \frac{\text{Weight of substance}}{\text{Molar mass} \times \text{Volume in liters}} \]

5. **Identifying the Correct Definition**:
- From the options provided:
- Number of gram equivalent per unit volume: Incorrect.
- Number of gram mole per liter: Correct.
- Amount of substance in gram per unit volume: Incorrect.
- Number of gram mole in 100 liters: Incorrect.
- Therefore, the correct definition of active mass is the "number of gram mole per liter".

### Final Answer:
Active mass is defined as the number of gram mole per liter.

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