Video Tutorial
Text:
A similar relative scale to the atomic mass scale could be developed
for gases using masses of equal volumes of gases as a base for
comparison. As gases exist as individual molecules, such a scale would
be termed the relative molecular mass scale.As before, a parallel scale measuring mass in grams will link to moles of gases.
For example:1 mole of hydrogen gas contains 6.02 x 1023 H2 molecules
1 mole of carbon dioxide gas contains 6.02 x 1023 CO2 molecules.
We shall see shortly that the relative molecular mass and relative atomic mass scale can in fact be thought of as a single scale. We will also be able to include masses of ions on such a scale.
Example 1:
How many moles of carbon dioxide gas will you have if you are told that you have 18.06 x 1023 molecules of the gas in your breath?
Solution:
First we must remember that there are 6.02 x 1023 particles in 1 mole of any substance. So we can write this problem as a division:
We can also relate the number of moles of a substance
to the mass of the substance using the relationship between the mole of
a substance and the relative atomic (or molecular) mass.
Example 2:
How many moles of hydrogen gas will be produced from a
reaction that evolved 40 grams of the gas? (Molecular mass of hydrogen
gas, H2 = 2.0.)
Solution:
Example 3:
An oxygen cylinder contains 6.0 moles of gas. If one
mole of oxygen gas has a mass 32.0g, what will be the mass of the gas in
the cylinder?
Solution:
Mass of oxygen = 6.0 mol. x 32.0 g per mole
= 192.0 g
Defining and relating the terms
To formalise and extend our understanding of relative
atomic mass and the mole we can give a definition based on our modern
understanding of the atomic mass scale.
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The relative atomic mass of an element is the ratio of the average mass of the atoms of the elements compared to the carbon−12 isotope that is allocated the mass of 12 units.
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A mole of a substance is equivalent in mass to the relative atomic mass or relative molecular mass in grams.
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The molar mass is the mass of one mole of a substance.