Avogadro's hypothesis predicts that at STP a one gram sample of Helium and a one gram sample of Nitrogen gas will occupy the same volume. A fixed quantity of a gas is subjected to a decrease in pressure and a increase in temperature. Suppose 65.38 grams of Zn are added to a concentrated hydrochloric acid solution and hydrogen gas is liberated according to the following equation. A sample of the gas occupying 2.241 L at 0 C and 1.00 atm is found to weigh 4.21 grams. The average speed of the molecules is directly proportional to the square root of the temperature of the sample and independent of their mass.
Molecules are in constant chaotic motion moving in straight lines between collisions.
The physical dimensions of the molecules occupy a very small fraction of the total system volume. All molecules are traveling at the same speed.