Dalton’s Law, simply stated, is that the total pressure exerted by a mixture of gases (real or ideal) in a fixed volume is equal to the sum of each gases partial pressure. Recall that the partial pressure of a gas is the pressure exerted by that gas on its container independent of any other gas. Because each gas in a mixture is at the same temperature and is contained in the same volume (i.e. E-cylinder) Dalton’s law states that their individual partial pressures can be simply added to find the total pressure in a container.
For example, consider an E-cylinder containing air at a pressure of 1900psi. Air is a mixture of roughly 20% oxygen and 80% nitrogen by volume. If we apply Dalton’s law we can say the partial pressure of oxygen plus the partial pressure of nitrogen is equal to the total pressure. Therefore, 20% of 1900psi is the partial pressure of oxygen exerted on the cylinder and 80% of 1900psi is the partial pressure of nitrogen exerted on the cylinder.