CHAPTER 9. GASES: Their Properties and Behavior. Pressure. force per unit area Standard Pressure 760 mm Hg 760 torr 76 cm Hg 1 atmosphere 29.925 inches Hg 101.3 kPa 1.013 bar. density Hg = 13.6 g/mL. Evangelista Torricelli observed the first vacuum in 1643. Boyle’s Law.
density Hg = 13.6 g/mL
Evangelista Torricelli observed the first vacuum in 1643
At a given temperature, the product of pressure and volume of a definite mass of gas is constant.
k1 = k2 (for same sample of gas at same T)
You must use the same units of pressure!
absolute zero = –273.15 °C
K = °C + 273 V = volume T = temperature in K
k1 = k2 (for same sample of gas at same P)
Combined Gas Law
This is one of two equations that must be memorized.
V1 = 750 mL T1 = 348 K T2 = 273 K
P1 = 810 torr P2 = 760 torr
T1 = 305 K V1 = 260 mL V2 = 500 mL
P1 = 0.5 atm = 380 torr P2 = 1200 torr
This could also be worked using 1200 torr = 1.58 atm
What is its density at STP?
Remember at STP 1 mol occupies 22.4 L
The combination of these three laws yields
This rearranges to give the Ideal Gas Law
R is a proportionality constant known as the Universal Gas Constant.
At STP and 1 mol of gas
The various R values will be given to you on the exams.
and the Combined Gas Law formulas
These two formulas will solve any problem involving gases.
T = 413 K P = 1820 torr = 2.39 atm
mass = 0.400 mol 16 (f.wt. CH4) = 6.40 g
Dalton’s Law of Partial Pressures
The total pressure exerted by a mixture of ideal gases is the sum of the partial pressures of the individual gases. (at constant T and V)
T (25 °C) and V (100 mL) are constant.
Since the pressure of each gas was measured in 100 mL, their pressures could be added. If the volume of either or both gases had changed, the pressure in that volume for a particular gas would need to be calculated before the values could be added together.
Therefore He effuses 4 faster than SO2.
The kinetic energy of a molecule is dependent upon its velocity (u).
The root-mean-squarevelocity of a molecule, urms, is calculated with the following formula
Example : Calculate the pressure exerted by 84.0 g of ammonia, NH3, in a 5.00 L at 200 °C using the van der Waal equation.
This is a 7.6% difference from the 38.4 atm calculated for this same problem using the ideal gas law.
The Chemistry of Explosions
2 (79 g) 2 (28 g) 4 (18 g) 32 g
~80 mL 44.8 L 89.6 L 22.4 L
The rapidly expanding gases are responsible for the explosive power. Consider that the average temperature of one of these explosions is >2500 °C (or 10 STP) and you glimpse the destructive power released.