Chemical Inertness of Nitrogen: Nitrogen exists as a diatomic molecule with an extremely strong triple covalent bond (), which makes it largely unreactive under standard conditions. It serves as a vital 'diluent' in the atmosphere, preventing rapid, uncontrollable combustion that would occur in a pure oxygen environment.
Oxidation Potential of Oxygen: Oxygen is a highly reactive non-metal that readily supports combustion and oxidation reactions. Its presence allows for aerobic respiration in living organisms and the formation of various oxides when it reacts with metals and non-metals.
Determination via Oxidation: The percentage of oxygen can be experimentally determined by reacting a known volume of air with an excess of a reactive substance, such as iron wool or phosphorus. As the oxygen is consumed to form a solid oxide (like rust or phosphorus pentoxide), the volume of the gas mixture decreases.
Measurement of Volume Contraction: By performing the reaction in a sealed apparatus, such as a burette or a bell jar inverted over water, the volume of oxygen removed can be measured by the rise in water level. The change in water level corresponds directly to the volume of oxygen that was originally present in the air sample.
Calculation Methodology: The percentage is calculated using the formula: This method assumes that the solid reactants do not take up significant volume and that all oxygen is fully reacted.
| Feature | Nitrogen () | Oxygen () | Carbon Dioxide () |
|---|---|---|---|
| Abundance | ~78% (Four-fifths) | ~21% (One-fifth) | ~0.04% (Trace) |
| Reactivity | Very Low (Inert) | High (Supports combustion) | Low (Greenhouse gas) |
| Biological Role | Protein synthesis (via fixation) | Aerobic respiration | Photosynthesis |
Account for Dead Space: In gas volume experiments, always check if there is an 'unmeasured' volume between the scale marks and the connection points (like the tap of a burette). Failing to add this 'dead space' to your initial volume calculation will lead to an overestimation of the oxygen percentage.
Ensure Completion: To get an accurate result, the reactant (iron or phosphorus) must be used in excess. If the reactant is the limiting factor, some oxygen will remain, and the calculated percentage will be lower than the true atmospheric value.
Consistency in Measurements: Always allow the apparatus to cool to room temperature before taking final volume readings, as gases expand when heated by the reaction (especially with phosphorus), which would yield incorrect volume data.
Confusing Nitrogen with Noble Gases: Students often mistake nitrogen for a noble gas because it is unreactive; however, nitrogen is a Group 15 non-metal with a diatomic structure, whereas noble gases like Argon are monatomic Group 18 elements.
Reaction Speed Errors: Oxidation reactions, particularly with iron wool, are slow and may take several days to reach completion. Taking a reading too early will result in an incomplete volume change and an incorrect percentage calculation.
Ignoring Water Vapour: Experimental calculations usually determine the percentage of oxygen in 'dry air.' In high-precision contexts, one must account for the fact that water vapour can displace other gases, though this is often simplified in basic laboratory settings.
