Question

How do you calculate the partial pressure of oxygen in the air?

Answer 1

Each constituent gas in a mixture of gases has a partial pressure, which is its hypothetical pressure if it alone occupied the whole volume of the original mixture at the same temperature. Dalton's Law states that the overall pressure of an ideal gas mixture is equal to the sum of the partial pressures of the gases in the mixture.

Complete answer:
The partial pressure of a gas is a measure of the gas's molecules' thermodynamic activity. Gases dissolve, disperse, and react in gas mixtures according to their partial pressures, not their concentrations. This common characteristic of gases is also true in biological chemical processes involving gases. For example, partial pressure of oxygen alone determines the amount of oxygen required for human respiration and the amount that is poisonous. This holds true for a wide variety of oxygen concentrations found in various inhaled breathing gases or dissolved in the blood. In examinations of arterial blood gases, partial pressures of oxygen and carbon dioxide are essential factors.
Two atoms of the element bond together at normal temperature and pressure to create dioxygen, a colourless and odourless diatomic gas with the formula ${O_2}$. Diatomic oxygen gas now makes up 20.95 percent of the Earth's atmosphere, however this has fluctuated significantly throughout time. Oxygen, in the form of oxides, makes up about half of the Earth's crust.
The component of total pressure exerted by a single chemical is partial pressure. It is proportional to the atmosphere's molar composition.
The partial pressure of oxygen would be ${\text{0}}{\text{.21x100 = 21kPa}}$ for an atmospheric pressure of 100 kPa and an estimated average oxygen content in the air of 21%.

Note:
The pressure of a vapour in equilibrium with its non-vapor phases is known as vapour pressure (i.e., liquid or solid). The word is most commonly used to describe the tendency of a liquid to evaporate. It is a measurement of a liquid or solid propensity for molecules and atoms to escape. The normal boiling point of a liquid is the temperature at which its vapour pressure equals the ambient atmospheric pressure, hence it is sometimes referred to as the atmospheric pressure boiling point.