What percentage of oxygen is in the air by weight?

The correct answer, which states that oxygen makes up approximately 21% of the atmosphere by weight, demonstrates the well-established composition of Earth's air.

Air is primarily composed of nitrogen (around 78%), oxygen (about 21%), with small amounts of other gases such as carbon dioxide, argon, and trace gases. The figure of 21% is significant because it indicates that while oxygen is vital for respiration and combustion, it is not the dominant gas in the atmosphere. Understanding the percentage of oxygen is crucial for various applications, including engineering, environmental science, and healthcare, as it impacts respiration rates, combustion processes, and overall atmospheric conditions.

This composition is a key aspect of atmospheric studies and has implications on various systems, including engines and HVAC systems, where the mixture of gases affects performance and efficiency.