## Partial pressure of water vapor

By Peter J. Mikulecky, Chris Hren. When gases mix, each individual gas within the mixture contributes a partial pressure, and adding the partial pressures yields the total pressure. This relationship makes sense if you think about pressure in terms of kinetic molecular theory. Adding a gaseous sample into a particular volume that already contains other gases increases the number of particles in that space. Because pressure depends on the number of particles colliding with the container walls, increasing the number of particles increases the pressure proportionally. She constructs an apparatus to combine pure samples of the primary volcanic gases that made up the atmosphere billions of years ago: carbon dioxide, ammonia, and water vapor. However difficult early-Earth atmospheric chemistry may prove to be, this particular problem is a simple one. The chemist inverts a test tube and immerses the open mouth into the reaction beaker to collect the hydrogen gas that bubbles up from the solution. The reaction proceeds to equilibrium. At the end of the experiment, the water levels within the tube and outside the tube are equal. The pressure in the lab is The vapor pressure of water at K is 3. What is the partial pressure of hydrogen gas trapped in the tube? The system has come to equilibrium, so the interior of the tube contains a gaseous mixture of hydrogen gas and water vapor. Because the water levels inside and outside the tube are equal, you know that the total pressure inside the tube equals the ambient pressure of the lab, The total pressure includes the partial pressure contributions from hydrogen gas and from water vapor. Christopher Hren is a high school chemistry teacher and former track and football coach. Peter J. About the Book Author Christopher Hren is a high school chemistry teacher and former track and football coach.

## How to Calculate Partial Pressures Using Dalton’s Law

Because it is dependent solely the number of particles and not the identity of the gas, the Ideal Gas Equation applies just as well to mixtures of gases is does to pure gases. In fact, it was with a gas mixture—ordinary air—that Boyle, Gay-Lussac, and Charles performed their early experiments. The only new concept we need to deal with gas mixtures is partial pressure, a concept invented by the famous English chemist John Dalton Dalton correctly reasoned that the low density and high compressibility of gases were indicative of the fact that they consisted mostly of empty space; from this, it Dalton concluded that when two or more different gases occupy the same volume, they behave entirely independently of one another. Mathematically, this can be stated as follows:. It is assumed that the gases do not react with each other. What is the total pressure inside the container? The mole fraction is a way of expressing the relative proportion of one particular gas within a mixture of gases. We do this by dividing the number of moles of a particular gas i by the total number of moles in the mixture:. What is the mole fraction of neon gas? The partial pressure of one individual gas within the overall mixtures, pican be expressed as follows:. A mixture of 2 mol H 2 and 3 mol He exerts a total pressure of 3 atm. What is the partial pressure of He? Consider a container of fixed volume We inject into that container 0. From the Ideal Gas Law, we can easily calculate the measured pressure of the nitrogen gas to be 0. We now take an identical container of fixed volume The measured pressure of the oxygen gas is 0. As a third measurement, we inject 0. Note that the mixture of gases we have prepared is very similar to that of air. The measured pressure in this container is now found to be 0. Our data show that the total pressure of the mixture of N 2 and O 2 in the container is equal to the sum of the pressures of the N 2 and O 2 samples taken separately. We now define the partial pressure of each gas in the mixture to be the pressure of each gas as if it were the only gas present. Our measurements demonstrate that the partial pressure of N 2 as part of the gas PN 2 is 0. Since gases have such small densities, it can be difficult to measure their mass.