Moles
Definition
• A mole is the mass of a substance
which contains the same number of
particles as 12 grams of the isotope
carbon12.
• These particles could be atoms, ions or
molecules.
• 12 grams of carbon12 contain 6.02 x1023
atoms
• This number is the Avogadro Constant,
and is given the symbol L.
Eg; One mole is…
63.5g of copper
16 + 2 = 18g of
water
27g of aluminium
Calculating the number of
particles
• Number of particles = moles x
Avogadro Constant.
• Eg; How many particles are there
in 0.1 moles of hydrogen?
• N = 0.1 x 6.02x1023 = 6.02 x 1022
molecules.
Calculating the number of moles
• Moles = number of particles /
Avogadro constant.
• Eg; How many moles of methane
are there in 3.01 x1023 molecules?
• Moles = 3.01x1023 / 6.02x1023 = 0.5
moles.
Calculating the molar mass
• One mole of an element has a mass
equal to the relative atomic mass in
grams.
• Eg; the mass of one mole of sodium is
23g.
• The mass of one mole of a compound
is found by adding together the masses
of all its elements in the correct ratio.
• Units gmol-1
• Eg; What is the molar mass of
calcium carbonate?
• Write the formula; CaC03.
• M = RAMCa + RAMC + 3xRAMO.
• M = 40 + 12 + 3x16 = 100g
Calculating the number of moles
• No moles = mass / RAM
• Eg; How many moles are there in
4g of calcium.
• Look up the mass number of
calcium in the periodic table.
• The atomic mass is the mass
number in grams.
• N = 4 / 40 = 0.1 moles
Think of the
equation as
a triangle
Mass (g)
m
n
Number of moles
RAM
Relative
atomic
mass
Rearranging;
n = m / RAM
Number of moles =
n
m
RAM
mass (g)
Relative
atomic
mass
Calculating the number of moles of a
compound.
• No moles = mass / Mr
• Eg; How many moles are there in 15g of
calcium carbonate?
• Look up the Ars, then add them
together to find the Mr.
• Mr = 40 + 12 + (16 x 3) = 100
• One mole has a mass of 100g.
• N = 15 / 100 = 0.15 moles.
Rearranging;
m = nRAM
m
n
Number of moles
RAM
X
RAM
mass =
Calculating the mass from moles
• Mass = No moles X Ar
• Eg; What is the mass of 0.75 moles of
sodium?
• Ar = 23
• Mass = 0.75 x 23 = 17.25g
• Similarly for a compound;
• Mass = No moles X Mr
• Eg; What is the mass of 0.8 moles of
sodium chloride?
• Mr = 23 + 35.5 = 58.5
• Mass = 0.8 x 58.5 = 46.8g