Download The Mole Concept and Avogadro`s constant

The Mole Concept and Avogadro's constant
The Mole-at first glace
 What is “the mole”?
 Dictionary definition: “a small burrowing mammal
with dark velvety fur, a long muzzle, and very small
eyes, feeding mainly on worms, grubs, and other
invertebrates…
 Animal?- must be biology-not chemistry!
 Are we in the right class?
The Mole-demystified
 Another definition-”Amount of
substance…”- more like chemistry!
Pair of socks
(2)
Dozen of eggs
(12)
 Ream of paper (500)
The mole & coins!
 Suppose you had to count a vast number of coins
 What is the quickest method?
 If the mass of a certain number of coins is known, the total number can be
weighed and then the amount can be calculated.
The Mole-demystified 2
 Quantity of 6.02x1023
 Why this magic number?
 It is the number of particles present in exactly 12 grams
of the isotope carbon-12
 Called the Avogadro Constant
The mole
 If you weigh out 12g of carbon-12, you will get 6.02x1023
of carbon atoms
The mole
 All elements measured relative to carbon-12
 Amount of substance (mol)= number of particles/6.02 x
1023mol-1
 Number of particles= amount of substance (mol) x 6.02 x
1023mol-1
 Mole is the same number regardless of the representative
particle: atom, molecule, ion, electron
The mole
Example 1
 Calculate the number of molecules of water in 0.01mol
of water
 Number of water molecules=0.01mol x 6 x 1023 mol-1 = 6
x 1021
 How many carbon atoms are there in 0.02 mol of
carbon dioxide?
 How many oxygen atoms are there in 0.02 mol of
carbon dioxide?
Molar Mass
 Molar mass is the mass of 1 mole of the substance
 Number of moles=mass in g/mass of 1 mole in g
 A mole of carbon atoms, C, has a mass of 12g
 A mole of hydrogen atoms, H, has a mass of 1g
 BUT a mole of hydrogen molecules has a mass of
2g
Molar mass- example
 Calculate the molar mass of aluminium sulfate,
Al2(SO4)3
 2 x Al= 2 x 26.98g/mol= 53.96g/mol
+3 x S= 3 x 32.06g/mol= 96.18g/mol
+12 x O= 12 x 16.00g/mol=192.00g/mol
Molar mass= 342.14g/mol
Relative atomic mass, relative
formula mass
 Relative atomic mass- the weighted average mass (according to relative
abundances) of all the naturally occurring isotopes of an element
compare with an atom of the carbon-12 isotope, which has a mass of
exactly 12
 Formula= 12 x average mass of one atom of the element/mass of one
atom of carbon-12
 Relative molecular mass is the sum of the relative atomic masses of all
the atoms in 1 molecule.
 Relative molecular mass=12 x average mass of one molecule of the
element/ mass of one atom of carbon-12
Molar mass vs relative
atomic/molecular masses
 Same except molar mass has units of g/mol
 Relative atomic mass & relative molecular mass are
dimensionless
Find the molar mass of magnesium carbonate, MgCO3
MgCO3= 24+12+3x16=84g/mol
Find the relative molecular mass of carbon dioxide, CO2.
CO2= 12+2x16=44 (NO UNITS!)
Amount of substance vs mass
More problems
 Find the moles of water molecules present in 54g of
water
 Moles of water molecules= 54g/18g mol-1=3.0mol
Everything combined!
 Calculate the number of calcium atoms present in
0.5kg 0f calcium
 Amount of calcium=500g/40g mol-1=12.5mol
 Atoms of calcium=12.5 mol x Avogadro's constant= 12.5
x 6.02 x 1023= 7.53 × 1024
Everything combined more!
 Calculate the number of carbon and hydrogen atoms and
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




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the total number of atoms in 22g of propane, C3H8.
Molar mass of propane, C3H8=(3x12)+(8x1)=44g mol-1
1 mole of propane therefore has a mass of 44g and contains
6.02x1023 molecules of propane.
Amount of propane=22g/44g mol-1= 0.50mol
Molecules of propane=0.5 x 6.02x1023=3x1023
Total number of atoms=11x3x1023=3.3x1024
Total number of carbon atoms=3x3x1023=9x1023
Total number of hydrogen atoms=8x3x1023=2.4x1024