Dalton's Law: If pO2 = 0.21 atm and pN2 = 0.79 atm, what is the total pressure of the gas mixture?

• A. 1.21 atm
• B. 1.00 atm
• C. 0.92 atm
• D. 1.50 atm

Answer: (B)

Dalton's Law says the total pressure of a gas mixture is the sum of the partial pressures of each component, assuming ideal behavior. Here, the partial pressure of oxygen is 0.21 atm and that of nitrogen is 0.79 atm. Adding them gives 0.21 + 0.79 = 1.00 atm. So the total pressure is 1.00 atm. The other numbers would not result from simply adding the given partial pressures, so they don’t reflect the correct application of Dalton’s Law. This also aligns with the familiar composition of air at roughly 1 atm total pressure.