Topic 1 - Matter and Energy - Revsworld

Topic 1 - Matter . and Energy
32. Avogadro’s law (hypothesis): Definitions, facts and example problems
Concept Fact: study to remember.
Avogadro’s law states that: Under the same conditions of temperature and pressure:
Equal volume of different gases contain equal number of molecules (particles).
Container A contains helium gas and container B contains oxygen gas.
NOTE that both containers have the same volume (2L) , and are at the same temperature and pressure.
Container A
Equal
Container B
2 L Volume
2 L
••••••••
••••••••
• 273 K • Temp
• 273 K
••••••••
••••••••
• 1 atm •••
Pressure
• 1 atm ••
••••••
••••••
• •
Helium gas
Oxygen gas
If the number of helium gas molecules are counted in Container A and the number of oxygen gas
molecules are counted in container B, you will find that:
The number of molecules of helium in A is the same as the number of molecules of oxygen in B.
Concept Task: Be able to determine which gas
sample has the same number of
molecules as a gas given in the question.
To determine which gas sample has the same
number of molecules as a gas given in the question
LOOK for a gas that has the same volume (L or mL)
Concept Task: Be able to determine which two gases
have the same number of molecules
To determine which two gas samples contain the
same number of molecules
LOOK for two gas samples with same temperature
and pressure
Example 21:
At STP, a 1.0 L sample of H 2 (g) would have
the same number of gas molecules as
1) 2.0 L of Ne 3) 0.5 L of He
2) 1.0 L of CO 4) 3.0 L of N 2
Answer: Choice 2. Because CO has the same
volume (1.0 L) as the volume in the
question.
Example 22:
The table below gives the temperature and
pressure of four different gas samples, each
in a 1.5 L container:
Gas sample Temperature (K) Pressure
(atm)
SO 2 200 of molecules: 1.5
Ar 300 3.0
N 2 200 1.5
O 2 300 1.5
Which two gas samples contain the
same number of molecules?
1) Ar and O 2 3) SO 2 and Ar
2) Ar and N 2 4) SO 2 and N 2
Answer: Choice 4 because both SO 2 and
N 2 gases have the same volume,
temperature and pressure
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