The percentage of carbon(IV) oxide in air is

1. Analyze the question

The question asks for the approximate percentage by volume of carbon(IV) oxide (also known as carbon dioxide, $CO_2$) present in the Earth's atmosphere.

2. Explain the concept

The composition of dry air is relatively constant. It is primarily composed of:

• Nitrogen ($N_2$): Approximately 78%
• Oxygen ($O_2$): Approximately 21%
• Argon ($Ar$): Approximately 0.9%
• Carbon(IV) oxide ($CO_2$): Approximately 0.03% to 0.04%
• Trace gases: Neon, Helium, Methane, etc.

In the context of standard chemistry curricula (like JAMB and WASSCE), the accepted value for the percentage of carbon(IV) oxide in air is typically taught as 0.03%.

3. Show the solution

• We know that air is a mixture of gases.
• Scientific data and standard chemistry textbooks used for Nigerian secondary examinations define the composition of $CO_2$ in the atmosphere as roughly 0.03% by volume.
• Comparing this to the provided options: • A (0.01%) is too low. • B (0.02%) is too low. • C (0.05%) is slightly higher than the standard accepted value. • D (0.03%) matches the standard textbook value.

4. Identify the answer

5. Tip

Exam Strategy: When dealing with questions about the composition of air, remember the "78-21-1" rule: roughly 78% Nitrogen, 21% Oxygen, and 1% for all other gases combined (which includes Argon at ~0.9% and $CO_2$ at ~0.03%). This helps you quickly eliminate options that are way off the mark!