1 st … Let’s Review Gases

In Lab, we often collect gas by water displacement:

When you’re done, there are many H 2 particles and some H 2 O particles.

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) P H2 gas = P total - P H2O 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 H 2 would be: mm Hg = 743 mm Hg (If you look up Water Vapor Pressure on Pg. 859, it’s 19 mm Hg at 21 C.)

Boyle’s Law and Charles’s Law

The Ideal Gas Law PV = nRT P = pressure in atm V = volume in liters n = moles of the gas T = temperature in Kelvin R = (gas constant)

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?

Solution: PV=nRT, isolate V V = nRT P n=2.12 moles

Solution (cont’) Temp in K? K = 33° + 273° = 306 K P in atm?

Now, plug in: So the balloon has 27.9 Liters in it.

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 N 2 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 N 2 ?

Solution: P 1 = 825 mm Hg = 825/760 = 1.09 atm P 2 = 8.5 atm T 1 = 25° + 273° = 298 K T 2 = ? 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:

Remove the variables which don’t change: Remove n and V:

Isolate the variable you want: Now isolate T 2 from the rest using the simple moving to the other side techniques:

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?