Dalton's law pertains to which concept?

• A. The total volume is equal to the sum of partial volumes
• B. The total pressure of a gas mixture equals the sum of the partial pressures
• C. Gas density is proportional to molecular weight
• D. Temperature remains constant in a closed system

Answer: (B)

Dalton's law deals with how pressures add in a gas mixture. In a container with several non-reacting gases, each gas would exert its own pressure as if the others weren’t there. The total pressure is the sum of those individual, or partial, pressures. Mathematically, P_total = P1 + P2 + P3, and each partial pressure equals the gas’s mole fraction times the total pressure (P_i = x_i × P_total).

This is why the correct concept is that the total pressure of a gas mixture equals the sum of the partial pressures. For example, in air at standard conditions, nitrogen and oxygen contribute portions of the overall pressure that add up to about 1 atm.

The other ideas don’t describe how pressures behave in a mixture: total volume isn’t defined as a sum of partial volumes, density isn’t what Dalton’s law addresses, and temperature remaining constant in a closed system is a separate consideration that doesn’t define how pressures combine.