Topic 1 - Matter and Energy - Revsworld

Topic 1 - Matter . and Energy
40. Gas law problem at constant Pressure: Example problem
Concept Task : Be able to solve gas law problems at constant pressure.
Example 26:
The volume of a confined gas is 25 ml at 280 K. At what temperature would the gas volume be 75 ml if
the pressure is held constant?
1) 140 K 2) 280 K 3) 560 K 4) 840 K
Before choosing a choice:
Step 1: Identify factors in the question: V 1 = 25 ml T 1 = 280 K
V 2 = 75 ml T 2 = X (unknown)
Step 2: Write down the combined gas law equation:
P 1 V 1 P 2 V 2
--------- = ----------
T 1 T 2
Step 3: Eliminate P (because Pressure is constant): V 1 V 1
------ = -------
T 1 T 2
Step 4: Set up problem by plugging in factors: 25 75
280 X
Step 5: Solve by: Cross-multiplying 25 X 21000
------- = -----------
Divide both sides by 25 25 25
Answer X = 840 K = T 2
Choose: Choice 4
As you can see, the new calculated temperature (840 K) is three times the initial temperature (280 K).
This makes sense because the new volume (75 ml) is also three times the initial volume (25 ml).
RECALL : At constant pressure, the volume is directly proportional to Kelvin temperature.
That is to say, if volume is tripled, the temperature is also tripled.
41. Gas law problem at constant volume: Example problem
Concept Task 18: Be able to solve gas law problems at constant volume.
--------- = --------
Example 27. NOTE: this problem is not a multiple choice problem
Pressure on a gas changes from 20 kPa to 50 kPa when the temperature of the gas is changed to 30 o C. If
volume was held constant, show set up, and calculate the initial temperature of the gas?
Step 1: Identify all factors in question: P 1 = 20 kPa T 1 = unknown
P 2 = 50 kPa T 2 = 30 o C (must be in Kelvin)
T 2 = 30 + 273 = 300 K
Step 2: Write down combined gas law equation: P____ 1 V 1 =
P 2 ____
V 2
T 1 T 2
Step 3: Eliminate V (Because volume is constant): ___ P 1 = ___ P 2
T 1 T 2
Step 4: Set up for T 1 by: Cross multiply factors P 2 T 1 = P 1 T 2
Divide both sides by P 2 T 1 = _____ P 1 T 2
Step 5: Substitute factors and solve for T 1 T 1 = (20) (300)
50
Answer T 1 = 120 K
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