Which of the following is not an expression of Dalton's law of partial pressures?

\(P = P_1 \chi _1\)

The correct answer is option 2. \(P = P_1 \chi _1\).

The expression \(P = P_1 X_1\) represents the partial pressure of a gas (\(P_1\)) multiplied by its mole fraction (\(X_1\)). This equation is actually derived from Raoult's law, which applies specifically to ideal solutions.

In Raoult's law, the partial pressure of a component in an ideal solution is equal to the vapor pressure of the pure component multiplied by its mole fraction in the solution. This law applies specifically to solutions, not to the mixture of non-reacting gases as described by Dalton's law of partial pressures.

So, the correct answer is (2) \(P = P_1 X_1\)