Composition and Separation of Air
This topic covers the composition of dry air and the industrial method used to separate its components. Key knowledge involves memorising the approximate percentages of the main gases and understanding the principles of fractional distillation as applied to liquefied air.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Dry air is a mixture of gases with a consistent composition. For the ESAT, you must know the approximate percentages by volume.
- Major components of dry air: Nitrogen (N2) is ~78%, Oxygen (O2) is ~21%, and Argon (Ar) is ~0.9%. Carbon dioxide (CO2) and other trace gases make up the remaining fraction.
- For calculation purposes, it's often sufficient to approximate the composition as 79% Nitrogen and 21% Oxygen, or even 4/5 Nitrogen and 1/5 Oxygen.
- The components of air can be separated by the fractional distillation of liquid air.
- This process involves cooling air to below -200°C under pressure to liquefy it. The liquid air is then warmed, and gases boil off at different temperatures according to their boiling points (N2: -196°C, Ar: -186°C, O2: -183°C).
- Nitrogen, having the lowest boiling point, boils first and is collected at the top of the fractionating column.
Formulae
Volume of a gas = (Percentage of gas / 100) × Total volume of air To calculate the volume of a specific component in a given sample of air. Remember that percentages are by volume.
Definitions
- Fractional Distillation
- A process for separating a liquid mixture into its individual components (fractions) based on their different boiling points. The substance with the lowest boiling point vaporises first, is collected, and then condensed.
Worked example
A 500 cm3 sample of dry air is passed over an excess of heated copper powder. The copper reacts with all the oxygen in the sample to form solid copper(II) oxide. What is the approximate volume of the gas remaining after the reaction, assuming temperature and pressure are constant?
- 1
Step 1:
Identify the gas that reacts.
The question states that copper reacts with oxygen.
- 2
Step 2:
Recall the percentage of oxygen in dry air, which is approximately 21%.
- 3
Step 3:
Calculate the volume of oxygen that reacts.
Volume O2 = 21% of 500 cm3 = (21 / 100) × 500 = 21 × 5 = 105 cm3 - 4
Step 4:
Calculate the remaining volume.
This is the initial total volume minus the volume of oxygen that was removed.
Remaining Volume = 500 cm3 - 105 cm3 = 395 cm3.
- 5
Step 5:
The remaining gas is primarily nitrogen and argon.
Answer: 395 cm3
Common mistakes
- ×Calculating the volume of gas removed (e.g., 105 cm3 of O2 in the example) and giving that as the final answer, instead of subtracting it from the total to find the volume *remaining*.
- ×Forgetting the context of 'dry air'. The percentages ~78% N2 and ~21% O2 apply to dry air. If a question implied humid conditions, water vapour would also be a component.
- ×Using a rough approximation like 20% for oxygen when a more precise calculation with 21% is straightforward and expected.
No-calculator tips
- ✓To calculate 21% of a number without a calculator, find 10% (divide by 10), double it to get 20%, then find 1% (divide by 100) and add it on. For 500: 10% is 50, so 20% is 100. 1% is 5. So 21% is 100 + 5 = 105.
- ✓To find the remaining gas volume (~79%) after oxygen is removed, it's often easier to calculate the 21% that is removed and subtract it from the total, rather than trying to multiply by 0.79 directly.
- ✓For quick estimations, think of air as 1/5 oxygen and 4/5 nitrogen. In the example, 1/5 of 500 is 100. The remaining gas would be 4/5 of 500, which is 400. This is a great way to check if your precise answer (395 cm3) is reasonable.