 In this video, I'm going to talk about one of the gas laws known as Dalton's Law. This is also known as Dalton's Law of Partial Pressures. But what Dalton's Law states is that the total pressure of a gas mixture is the sum of the partial pressures of the gases in the mixture. The formula for Dalton's Law is P total, which is the total pressure of the system, is equal to the pressure of your first gas, or the partial pressure of the first gas, plus the partial pressure of the second gas, plus the partial pressure of the third gas, and so on, depending on the number of gases present in your mixture. But basically, what Dalton's Law states is that if I have a container that contains helium gas, and this helium gas has a pressure of five atmospheres, and I have a second container, this time with hydrogen gas, which has a pressure of two atmospheres. If I were to put both of these gas samples into the same container, then my overall pressure of my helium plus hydrogen gas mixture would be seven atmospheres, being the partial pressure of the helium, partial pressure of your first gas, plus the partial pressure of the hydrogen, your second gas, the sum of those would be the total pressure of your gas mixture. In this case, it would be seven atmospheres. So it doesn't matter what type of gas it is. Pressure is not related on the type of gas in your mixture. Pressure has to do with how many particles are present, and the speed at which these particles are colliding with the surface of the container. Now how would we use Dalton's law in an example problem? Here I have a problem which states that we have a gas mixture containing oxygen and argon gas. The partial pressures of these gases are 0.30 atmospheres for the oxygen gas and 2.3 atmospheres for the argon gas. If we add a third gas to our mixture, nitrogen gas, and the total pressure of the mixture increases to 3.6 atmospheres, we want to calculate what is the partial pressure of the nitrogen gas that we added. So in order to calculate the partial pressure of nitrogen, we first need to look at the problem and determine what's been given to us. We know that the total pressure on the system after the nitrogen has been added is 3.6 atmospheres. The problem also tells us that the partial pressure of the oxygen gas is equal to 0.3 atmospheres. And it tells us that the partial pressure of the argon gas is 2.3 atmospheres. Now all that you have to do is insert these variables into our Dalton's law equation, and then we're going to solve for the unknown variable. So our P total is equal to 3.6 atmospheres. And that is going to be equal to the sum of our partial pressures, which is 0.3 plus 2.3 plus the partial pressure of our nitrogen gas, which is the unknown variable that we want to solve for. In order to solve for the partial pressure of the nitrogen gas, we need to move the 2.3 and the 0.3 over to the left side of the equal sign. In order to do that, if we add 2.3 atmospheres on the right side, we can subtract 2.3 atmospheres. And those variables will cancel out. But what we do to one side, we have to do the other side. So we're going to subtract 2.3 atmospheres from the left side as well. We're going to do the same thing with the 0.3 atmospheres. We subtract it from the right side. We subtract it from the left side. Whenever we take our 3.6 and we subtract 2.3, and then we subtract 0.3, our partial pressure of our nitrogen is equal to 1 atmospheres.