10.03.03 The Mole (HT)
Syllabus
Chemical amounts are measured in moles.
The symbol for the unit mole is mol.
The mass of 1 mole of a substance (grams) is numerically equal to its relative formula mass.
One mole of a substance contains the same number of the stated particles, atoms, molecules or ions as one mole of any other substance.
The number of atoms, molecules or ions in a mole of a given substance is the Avogadro constant. The value of the Avogadro constant is 6.02 x 1023 per mole.
Students should understand that the measurement of amounts in moles can apply to atoms, molecules, ions, electrons, formulae and equations, for example that in one mole of Carbon (C) the number of atoms is the same as the number of molecules in one mole of Carbon Dioxide (CO2).
Students should be able to use the relative formula mass of a substance to calculate the number of moles in a given mass of that substance and vice versa.
What does this mean?
In the previous section we saw how to work out an Mr.
And we said that its Mr is equal to the mass of 1 mole.
But what is a mole?
We know CH4(g) + 2 O2(g) --> CO2(g) + 2 H2O(g) means that 1 Methane(CH4) molecule needs 2 Oxygen molecules to burn, making 1 CO2 molecule and 2 molecules of Water.
But we never have one molecule of anything.
So this equation actually means that 1 mole of Methane (CH4) needs 2 moles of Oxygen to burn making 1 mole of CO2 and 2 moles of Water.
But how many molecules is this?
The Avogadro number (6 x 1023) is the number of molecules of Methane in 1 mole of Methane
This means there are 600,000,000,000,000,000,000,000 methane molecules in a mole of Methane (16g)
But 1 molecule of Methane (CH4) contains 1 C atom and 4 H atoms.
So, 1 mole of Methane contains 1 mole of Carbon Atoms and 4 moles of Hydrogen atoms.
In other words 6 x 1023 Carbon atoms and 24 x 1023 Hydrogen atoms.
Questions
1. How many atoms in 2 moles of Helium ?
Helium exists as individual atoms.
So, 1 mole of Helium is 6 x 1023 atoms.
2 mole of Helium is 12 x 1023 atoms.
Or 1.2 x 1024.
2. How many atoms in 3 moles of Oxygen (O2)?
Oxygen exists as diatomic molecules.
So, 1 mole of Oxygen is 2 x 6 x 1023 atoms = 1.2 x 1024 atoms.
3 mole of Oxygen is 3 x 1.2 x 1024 = 3.6 x 1024 atoms.
3. How many atoms in 5 moles of Ethane (C2H6)?
Ethane molecules contain 2 Carbon and 6 Hydrogen atoms = 8 atoms each.
So, 1 mole of Ethane is 8 x 6 x 1023 atoms = 4.8 x 1024 atoms.
5 mole of Ethane is 5 x 4.8 x 1024 = 2.4 x 1025 atoms.
3. How many atoms in 10 moles of Buckminsterfullerene (C60)?
Buckminsterfullerene molecules contain 60 atoms each.
So, 1 mole 60 x 6 x 1023 atoms = 3.6 x 1025 atoms.
10 moles is 10 x 3.6 x 1025= 3.6 x 1026 atoms.
1. There are 12,000,000,000 atoms of Neon in a jar. How many moles of atoms is this?
1 mole is 6 x 1023 atoms.
Moles of Neon = 12,000,000,000 / 6 x 1023
= 2 x 10-14 moles.
2. There are 3 x 1013 molecules of Oxygen in a container. How many moles of Oxygen is this?
1 mole is 6 x 1023 molecules.
Moles of Oxygen = 3 x 1013 / 6 x 1023 = 0.5 x 10-10
= 5 x 10-11 moles.
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