"Good morning. Today, we will be discussing
Dalton's Law of Partial Pressures. I do hope you all have been doing your readings."
Dalton's Law of Partial Pressure:
The pressure of a mixture of gases is equal to the sum of the pressures of all of the constituent gases alone.
Mathematically, this can be represented as:
PressureTotal = Pressure1 + Pressure2 ... Pressuren
Explanation and Discussion:
Dalton's Law explains that the total pressure is equal to the sum of all of the pressures of the parts. This only is absolutely true for ideal gases, but the error is small for real gases. This may at first seem a trivial law, but it can be very valuable in the chemistry lab.
Let's say you want to collect hydrogen gas. To do this, you set up a system that uses a pneumatic trough, a test tube that has a pipetted stopped, a cable that connects the pipett into the pneumatic trough, and a test tube above the cable that collects the hydrogen. Warning: Do not conduct this experiment unless you are under the direction of a chemist or your chemistry teacher. It is dangerous and involves a Bunsen burner and dangerous materials. You submerge the test tube that will collect the hydrogen, and tilt it up so it only contains water. By placing zinc and acid in the pipetted test tube and heating it, hydrogen gas is given off. This gas pumps through the water and enters into the collection test tube. After the first few seconds, the gas will be pure hydrogen. Image of start of hydrogen generation. When the water level is equal in the test tube and the trough, turn off the generator. The pressure inside the test tube will be equal to the atmospheric pressure. Image of pressure equalibrium in hydrogen generator. Now you can use the ideal gas law to determine the number of hydrogen moles in the test tube, right? Not quite.
You see, the water you collected the hydrogen over has vapor pressure that will distort the equation if not accounted for. Because of the Dalton's Law of partial pressure, you know that the pressure in the test tube is from both the hydrogen and the water. To find just the hydrogen, you would have to subtract the vapor pressure of the water. Vapor pressure of water is published in most chemistry books as a table in the appendix, and varies by the temperature of the water.
"On Thursday, we will begin using calculations with Dalton's Law. Be prepared for a quiz next Tuesday."