Final answer:
In a gas of molecular mass m, velocity is the parameter directly proportional to the square root of the temperature, as described by Maxwell's distribution law.
Step-by-step explanation:
According to Maxwell's distribution law, which describes the distribution of molecular speeds at a given temperature, in a gas of molecular mass m, the parameter that is directly proportional to the square root of the temperature is the velocity of gas molecules. This relationship is due to the temperature of a gas being proportional to the average translational kinetic energy of its molecules, which relates to velocity through the square root of temperature based on the equation for kinetic energy. Therefore, as temperature increases, the typical speed or root-mean-square velocity (Urms) of gas molecules increases as well.