Question

Two gas jars are connected to each other, but they are separated by a closed valve. One gas jar contains oxygen, and the other contains hydrogen. What will happen when the valve is opened?

Answer 1

Both gasses will diffuse into the other jar until the molecules are well distributed into both containers. Diffusion is the movement of molecules from a point of high concentration to areas of low concentration until equilibrium is attained.   The mixture of gases will have a partial pressure that is equal to the sum of the partial pressures of the component gases.