Unit 3, Lessons 15 and 16

Lesson 15:

A plane in flight gains elevation after lifting off the runway. Passengers in the plane's pressurized cabin don't feel the change, but outside the plane, the pressure and temperature of the air decreases and breathing becomes difficult. There aren't as many molecules at high elevations to support life or respiration, so gases are lighter and, ultimately, colder. Hence, the relationship between altitude, pressure, and density. The number of molecules, however many there may be, can be described by the term number density, which is represented by the following formula:

n/V

where n is the number of moles--a fancy term used to describe how many atoms there are in a sample of gas. 

Problems:

2.) Use the kinetic theory of gases to explain the relationship between number density and gas pressure. As I stated in my summary of this lesson, as pressure increases, there are more molecules condensed into a space, and vice versa. If the pressure drops, there are less molecules. The molecules don't change the speed at which they fly, but there are less of them, which means they don't bump into one another/change direction as often.

Lesson 16:

Our friend the mole is represented by the number 6.022x10^23, or 602 sextrillion--huge! Moles are measurements of how many atoms, particles, whatever-you-want-to-call-them there are in a sample of gas. If you don't know how many moles are present, use this nifty equation to figure it out:

PV = nRT

where P = pressure, V = volume, n = moles, R = the constant (0.082) for all gases, and T = tempterature (in  Kelvin.) Though any value can be plugged in for those variables to find another, it's more common to see the equation used at STP, the standard temperature and pressure, which are 273 K and 1.0 atm.

Problems:

5.) Which has more atoms: 8.0 g of helium, He, or 40.0 g of argon, Ar? Explain. Both of these elements are noble gases, but argon is in a larger quantity, so I'd say 40.0 g of Ar has more atoms.

7.) At 25 degrees C, which balloon has the greater volume: an oxygen, O2, balloon at 1.2 atm with a mass of 16.0 g, or a helium, He, balloon at 1.2 atm with a mass of 2.0 g? By Avogadro's law, two samples of gas at the same temperature and pressure are said to have the same number of particles regardless of their mass. This, in turn, should make their volumes the same. The two balloons have an equal volume, but you'll need 8x as more He to equal O.