Chapter 11 Molecular composition of gases
Section 1
In 1808, Gay-Lussac did some experiments that showed that the volume of gaseous reactants and products can be expressed as ratios of small whole numbers, as long as the gases are at constant temperature and pressure. Avogadro was able to add to this be rejecting Dalton's idea that the reactant elements are always in monatomic form. Avogadro said that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.
It is very important to remember that the gases have to be at the same temperature and pressure. We usually use Standard Temperature and Pressure. STP is important. At STP, the volume occupied by a one mole of gas is 22.4 Liters. That is another ratio/fraction.
It is 22.4L/1 mole. Now we can change Liters of a gas into moles, as long as the gas is at STP. We can also change moles of a gas into Liters, as long as the gas is at STP.
Section 2
But what if the gas is not at STP? We know that this is going to happen. Along comes the Ideal Gas Law. It combines Boyles, Charles and Gay-Lussac's laws.
The equation is PV=nRT "P" is still pressure, and is measured as atm, mmHg, or kPa. "V" is still volume, measured in Liters or M3 or parts of a Liter or M3 like cm3 or mL. "T" is still temperature, and is always converted to Kelvin's. "n" means number of moles. Here it is again!! The mole is back. Now we can convert amounts of grams to moles, or even find the amount of grams.
But wait! What is the "R"? "R" is the symbol for the Ideal Gas Constant. The number we substitute for the "R" depends on how we measured the pressure. If pressure is measured in atm, then use 0.0821. If pressure is measured in mmHg, use 62.4. If pressure is measured in kPa, use 8.314 I will go over the rest of the labels for "R." Basically "R" is used to cancel labels. It is pretty handy.
You will need to come to class to see how we work these
problems. It is very hard to show all the variations that will
occur. We can find molar mass, which is grams/mole, and we can find
density, which is mass/volume, and we can even find the grams of a cpd.
Section 3
This is the stoichiometry again.
As long as a gas is at STP, we can use the same ratio method that we have always used. It is just a matter of matching the fractions.
However, if the gas is not at STP, we will use PV=nRT.
Once again, we will need the equations, and you will have to match the numbers to the correct formula. It will be a big help to be in class so that you can see how we decide how to work out the problems.
Section 4
Graham's Law of Effusion basically says that the square root of the molar mass of the heavier gas over the square root of the molar mass of the lighter gas will give you the RATE at which the two gases will effuse.