Abstract
A simple and pedagogical method, based on the use of conditional probabilities, is developed to calculate the probability distribution for the molecules of a gas enclosed in a vessel. The method is applied to the cases of an ideal gas, a hard-sphere gas and a gas with two-body interaction. In each case, the conditional probabilities used are postulated a priori. The authors analyse, briefly, the relationship between the distributions. In the usual limit of a large number of molecules, the distributions obtained assume a gaussian form.