Why is nitrogen prepared from air denser than pure nitrogen

General
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why is nitrogen prepared from air denser than pure nitrogen

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Why is nitrogen prepared from air denser than pure nitrogen?

Answer:
@Ozkanx, nitrogen prepared from the air is denser than pure nitrogen because it often contains impurities. These impurities, which may include other gases such as oxygen, argon, carbon dioxide, and traces of other elements, influence the overall density of the gas mixture.

1. Composition of Air-Derived Nitrogen

  • Air Composition:
    • Air is a mixture of gases, predominantly nitrogen (78%), oxygen (21%), and small amounts of other gases such as argon, carbon dioxide, and water vapor.
  • Separation Process:
    • When nitrogen is separated from the air through processes such as fractional distillation, there can still be remnants of other gases, primarily oxygen and argon.

2. Density Differences due to Impurities

  • Pure Nitrogen:
    • Pure nitrogen has a molecular weight of approximately 28 g/mol.
  • Air-Derived Nitrogen:
    • Air-derived nitrogen can contain small amounts of oxygen (molecular weight of approximately 32 g/mol) and argon (atomic weight of approximately 40 g/mol), increasing the average molecular weight of the mixture.
    • Even in trace amounts, these heavier gases can make the mixture slightly denser than pure nitrogen.

3. Calculation Example:

To understand this better, let’s consider an example:

  • Molecular Weight Calculations:
    • If pure nitrogen (molecular weight = 28 g/mol) is entirely pure, the density calculation will be straightforward.
    • Suppose air-derived nitrogen contains 98% nitrogen, 1.5% oxygen, and 0.5% argon by volume, then the average molecular weight can be calculated as follows:
\text{Average Molecular Weight} = (0.98 \times 28) + (0.015 \times 32) + (0.005 \times 40)
\text{Average Molecular Weight} = 27.44 + 0.48 + 0.20 = 28.12 \text{ g/mol}
  • Result:
    • This example shows that the presence of even small amounts of oxygen and argon increases the average molecular weight of the nitrogen mixture, making it slightly denser than pure nitrogen.

4. Practical Implications:

  • Industry Standards:
    • In industries where highly pure nitrogen is required, additional purification processes such as catalytic deoxygenation or pressure swing adsorption (PSA) may be used to remove impurities and achieve nitrogen purity levels above 99.99%.
  • Scientific Applications:
    • The density differences, albeit slight, can impact certain scientific or industrial applications where precision and purity of gas are crucial.

Final Answer:
Nitrogen prepared from air is denser than pure nitrogen due to the presence of impurities like oxygen and argon which have higher molecular weights. These impurities, even in small amounts, increase the overall density of the nitrogen mixture.