Dalton's law of partial pressures states that the total pressure of a gas mixture is equal to:

• A. Ptot = P1 x P2 x P3
• B. Ptot = P1 / P2
• C. Ptot = P1 - P2
• D. Ptot = P1 + P2 + P3

Answer: (D)

Partial pressures from each gas in a mixture add to give the total pressure. In an ideal gas mixture, each gas contributes P_i = n_i RT / V, so the total pressure is P_tot = sum of the individual partial pressures = P1 + P2 + P3 for three gases. This additive behavior comes from the independent, non-interacting motions of the gas molecules. If you use mole fractions, P_i = x_i P_tot and since sum of x_i equals 1, you still get P_tot as the sum of the partial pressures. The other mathematical forms don’t capture this independent, additive contribution of each gas to the total pressure. This relies on ideal gas behavior and no chemical reactions among the gases.