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Physical Property: Density
Density is mass per unit volume of a substance, which also determines whether a substance floats or sinks in a medium.
Density = Mass / Volume
Using density to explain how the methods work:
When we collect gases, we will always compare the density of the gases to the surrounding air:
When gases are less dense than air, the gas will float.
These gases upward delivery of gases for collection
When gases are denser than air, the gas will sink.
These gases require downward delivery of gases for collection
Examples of Gases using each method:
Based on the density of gases and air, we can remember some examples of gases for each method:
Gases that are less dense than air, and requires the upward delivery of gases
Examples: hydrogen, helium, ammonia
Gases that are denser than air, and requires the downward delivery of gases
Examples: carbon dioxide, oxygen
Trick: Using Periodic Table to determine molecular masses of elements and compounds to determine density
As Density = Mass / Volume, we can find the atomic mass (mass of atom) or molecular mass (mass of molecule) of various gases to predict whether these gases will have high or low density. We can do this by looking at the mass (nucleon) number of the elements in the Periodic Table.
2 hydrogen
Molecular Mass of Hydrogen Gas
= 2 x 1.0
= 2.0 g
(Using Periodic Table)
low mass, low density, floats
upward delivery of gas
3 hydrogen, 1 nitrogen
Molecular Mass of Ammonia Gas
= 3 (1.0) + 14
= 17.0
(Using Periodic Table)
low mass, low density, floats
upward delivery of gas
2 oxygen
Molecular Mass of Oxygen Gas
= 2 x 16.0
= 32.0
(using Periodic Table)
high mass, high density. sinks
downward delivery of gas
1 carbon, 2 oxygen
Molecular Mass of Carbon Dioxide Gas
= 12.0 + 2(16.0)
= 44.0
(using Periodic Table)
high mass, high density, sinks
downward delivery of gas
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