Active mass, in the context of Class 11 chemistry, refers to the effective concentration of a reacting substance that participates in a chemical reaction. It's not just about the total amount present, but the amount that is actively involved. In practical terms, active mass is synonymous with molarity, which measures concentration in moles per liter.
Understanding Active Mass
Active mass plays a crucial role in determining the rate of a chemical reaction. According to the reference, it is calculated as follows:
- Active mass = n/V
Where:
- n represents the number of moles of the substance.
- V represents the volume of the solution in liters.
Key Points about Active Mass:
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Molarity: Active mass is essentially the molar concentration or molarity of a substance. This is the number of moles of a substance present in one liter of the solution.
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Reacting Portion: It refers specifically to the portion of a substance that can participate in a reaction. For example, in a solution of an electrolyte, the active mass is the concentration of ions that are dissociated from the electrolyte and able to conduct electricity.
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Units: The units for active mass are moles per liter (mol/L) which can also be represented by the unit M, for molarity.
Calculation Steps:
To calculate active mass, follow these steps:
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Determine the Number of Moles (n):
- If you are given the mass of the substance, divide it by its molar mass to get the number of moles.
- Formula: moles = mass (g) / molar mass (g/mol)
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Determine the Volume (V):
- The volume of the solution must be expressed in liters. If given in milliliters (mL), convert it to liters by dividing by 1000.
- Formula: Volume in liters (L) = Volume in milliliters (mL) / 1000
- The volume of the solution must be expressed in liters. If given in milliliters (mL), convert it to liters by dividing by 1000.
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Calculate Active Mass:
- Divide the number of moles (n) by the volume in liters (V) as per the formula.
- Formula: Active mass = n/V
- Divide the number of moles (n) by the volume in liters (V) as per the formula.
Examples
Example 1:
- Suppose you have 10 grams of NaOH dissolved in 250 mL of water. Calculate the active mass.
- Moles (n): Molar mass of NaOH is 40 g/mol. Moles of NaOH = 10 g / 40 g/mol = 0.25 mol
- Volume (V): Volume in liters = 250 mL / 1000 = 0.25 L
- Active Mass: Active mass = 0.25 mol / 0.25 L = 1 mol/L (or 1 M)
Example 2:
- If 0.1 moles of a substance are dissolved in 500 mL of water. Calculate the active mass.
- Moles (n): 0.1 mol
- Volume (V): 500 mL = 0.5 L
- Active Mass: Active mass = 0.1 mol/ 0.5 L = 0.2 mol/L (or 0.2 M)
Summary
| Step | Description | Formula |
|---|---|---|
| 1. Calculate Moles | Convert mass to moles if needed using the molar mass. | n = mass / molar mass |
| 2. Convert Volume to Liters | Ensure solution volume is in liters; convert from mL if necessary. | V (L) = V (mL) / 1000 |
| 3. Calculate Active Mass | Divide moles by the volume in liters to find molarity (active mass). | Active mass = n / V |
By calculating the active mass, one can understand the concentration of a substance, which is crucial for predicting the behavior and rate of a chemical reaction. The concept, explained above, is also relevant in the dissociation of electrolytes. As mentioned, the active mass also represents the concentration of the portion of a dissolved electrolyte that dissociates into ions, thereby influencing the electrical conductivity of the solution.
