Partial Pressure Definition and Real Life Examples

Partial pressure is a term used in chemistry and physics to describe the pressure exerted by a particular gas in a mixture of gases. It represents the hypothetical pressure the gas would exert if it occupied the entire volume of the mixture on its own, at the same temperature.

In a mixture of gases, each gas contributes to the total pressure in proportion to its concentration or mole fraction in the mixture. The partial pressure of a specific gas is calculated by multiplying its mole fraction (the ratio of the number of moles of that gas to the total number of moles of all gases in the mixture) by the total pressure of the gas mixture. Mathematically, the partial pressure (Pi) of a specific gas I in a gas mixture is given by:

Pi=x_i × P_total

where:

• x_i is the mole fraction of the gas i in the mixture
• P_total is the total pressure of the gas mixture

Real Life Examples

1. When you breathe in, the air is a mixture of gases, and each gas exerts its partial pressure.
2. Carbonated drinks contain dissolved carbon dioxide, contributing to their fizziness through partial pressure.
3. Inflating a tire involves adding a gas, and each gas in the mixture exerts its partial pressure.
4. Scuba divers breathe a gas mixture in their tanks, where each gas has a specific partial pressure.
5. Cooking with a gas stove involves a gas mixture, and each gas contributes to the overall partial pressure.

Exam Related Questions

No.	Question	Answer
1.	What is partial pressure?	Partial pressure is the pressure exerted by a specific gas in a mixture of gases. It represents the hypothetical pressure the gas would exert if it occupied the entire volume on its own at the same temperature.
2.	How is partial pressure calculated?	The partial pressure of a gas in a mixture is calculated by multiplying the mole fraction of the gas by the total pressure of the mixture. Mathematically, Pi=xi×Ptotal, where xi is the mole fraction of the gas, and Ptotal is the total pressure of the gas mixture.
3.	Why is partial pressure important in gas mixtures?	Partial pressure is crucial for understanding and predicting gas behaviours, especially in gas mixtures and chemical reactions. It helps in analyzing gas properties and behaviors under different conditions.
4.	Calculate the partial pressure of oxygen	Given a mixture of air with a total pressure of 1 atm and an oxygen mole fraction of 0.21, the partial pressure of oxygen (PO2) can be calculated as PO2=0.21×1 atm=0.21 atm
5.	Calculate the partial pressure of nitrogen	In the same air mixture, with a nitrogen mole fraction of 0.79, the partial pressure of nitrogen (PN2​) can be calculated as PN2=0.79×1 atm =0.79 atm
6.	Calculate total pressure in a gas mixture	If the partial pressures of oxygen and nitrogen in a gas mixture are known (e.g., PO2=0.21 atm and PN2=0.79 atm), the total pressure (Ptotal) can be calculated by summing the partial pressures: Ptotal=PO2+PN2=0.21 atm+0.79 atm=1 atm

Real Life Scenario

let’s consider a scenario where we are breathing in air at sea level, where the total atmospheric pressure is approximately 1 atmosphere (atm). The composition of dry air at sea level typically includes about 21% oxygen and 78% nitrogen, with trace amounts of other gases making up the remaining 1%.

1. Oxygen Partial Pressure:
   • Oxygen constitutes about 21% of the air. So, the partial pressure of oxygen (PO₂) is approximately 0.21 atm (21% of 1 atm).
2. Nitrogen Partial Pressure:
   • Nitrogen constitutes about 78% of the air. Therefore, the partial pressure of nitrogen (PN₂) is approximately 0.78 atm (78% of 1 atm).
3. Total Atmospheric Pressure:
   • The sum of the partial pressures of oxygen and nitrogen gives us the total atmospheric pressure: P_total=PO₂+PN₂
     P_total=0.21 atm+0.78 atm
     P_total≈0.99 atm

This total pressure of about 0.99 atm approximates the atmospheric pressure at sea level.

References

1. “Chemistry: The Central Science” by Theodore L. Brown, H. Eugene LeMay, Bruce E. Bursten, and Catherine Murphy.
2. “Principles of General Chemistry” by Martin S. Silberberg.
3. “Physical Chemistry: A Molecular Approach” by Donald A. McQuarrie and John D. Simon.

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