To get the pressure of just the hydrogen in the flask:
• You subtract the partial pressure of the water vapor from the total pressure in the flask. (Table on page 859)
• PH2 gas = Ptotal - PH2O vapor • IE: If the pressure in the room is 762 mmHg and
the temp. in the room is 21 C, then the partial pressure of just H2 would be:
• 762-19 mm Hg = 743 mm Hg• (If you look up Water Vapor Pressure on Pg. 859,
it’s 19 mm Hg at 21 C.)
The Ideal Gas LawPV = nRT
• P = pressure in atm
• V = volume in liters
• n = moles of the gas
• T = temperature in Kelvin • R = 0.0821 (gas constant)Kmol
atmL
There are two kinds of gas law problems:
• 1. Solve for one variable in PV=nRT in the same conditions
• 2. Solve for a variable after a change in the conditions (a change in P, V, n, or T)
How to Solve for one variable:
• 1. Manipulate PV=nRT to isolate the variable you need using algebra
• 2. Convert all units to L, atm, moles, & K and plug in the values to solve for the isolated variable.
• A balloon is filled with 34.0 g of methane. The temperature is 33 C and the pressure inside the balloon is 1450 mm Hg. What is the volume of the balloon?
Now, plug in:
• So the balloon has 27.9 Liters in it.
Litersatm
KKmol
atmLmols
P
nRTV 9.27
91.1
)306)(0821)(.12.2(
A sample problem:
• A 44.0 gallon steel barrel with some water in it is heated to 300 ºC. The pressure is measured to be 7.82 atm.
• How many grams of water are in the barrel?
• Assume no other gas is in the barrel. Also, 1 gallon is 3.78 Liters.
Before & After Problems:
• Look for words & numbers that indicate a change has taken place. The best way is to write out all the variables using subscripts 1 & 2 to show if it’s before or after.
Example:
• A sample of N2 gas has a pressure of 825 mm Hg at 25 ºC. What temperature is required to increase the Pressure to 8.5 atm, assuming fixed volume & amount of N2?
Solution:
• P1 = 825 mm Hg = 825/760 = 1.09 atm
• P2 = 8.5 atm
• T1 = 25° + 273° = 298 K
• T2 = ?
• This means that n & V don’t change
• R never changes.
Here’s an easy way to isolate what you need:
• Always start with this, since:
22
22
11
11
TnVP
TnVP
R
Isolate the variable you want:
• Now isolate T2 from the rest using the
simple moving to the other side techniques:
Katm
KatmPTP
T 230009.1
)298)(5.8(
1
122
Sample Problem:
• A 3.50 mL bubble of air forms deep in a lake where the Temp. is 7.0 C and the Pressure is 1.95 atm.
• The bubble rises to just below the surface of the lake, where the Temp. is 18 C and the pressure is 750 mm Hg.
• What’s the volume of the bubble just before it hits the surface of the lake?