Y11 DP Chemistry
R. Slider
As you know, atoms are very, very small…
Some single atom masses:
Li: 1.15217 x 10-23 g
O: 2.65659 x 10-23 g
He: 6.64605 x 10-24 g
U: 3.95233 x 10-22 g
These masses are not terribly convenient to work with, so chemists work with
relative masses. The relative atomic masses of all the elements are based on the
mass of C-12 (1.99X10-22 g), the most abundant isotope on the Earth. In fact:
Relative atomic mass (Ar) is defined as the mass of an
element relative to 1/12 of the mass of an atom of the
Carbon-12 isotope.
These relative atomic masses are conveniently located on the Periodic Table
and have no units since they are all relative. H: 1.01 P: 30.97 Br: 79.91
The mole is a unit used only in
Chemistry that denotes the amount
of substance (n).
Since chemists had determined the
relative atomic masses of the elements, it
logically followed that 1.01g of H had
the same number of atoms as 12.0g of C.
And that same number of particles is
equal to a mole of that substance.
This is also a mole…
The Mole is…
The number equal to the number of carbon atoms in exactly 12 grams of
pure 12C. (We once compared to H, the lightest element. Now we use C-12,
the most abundant)
 1 mole of anything = 6.022  1023 units of that thing (atoms, ions,
molecules, grains of sand, etc.)
12g of C-12 contains 6.022x1023 atoms.
This is known as Avogadro’s
Number (NA or L) and is equivalent
to 1 mole of carbon atoms.
Avogadro’s number = 6.02x1023 = 1mole
Notice that the mass of 1 mole of C-12 is
the same value as the relative atomic mass
for C-12 on the Periodic Table.
In the same way, 1 mole of any element or
compound is equivalent to its atomic,
molecular or formula weight.
We can now define the relative atomic
masses from the periodic table as molar
masses with the units g/mol.
Substance
Molar mass
(g/mol)
carbon
12
chlorine
oxygen
35.5
16
water
18
Molar mass = mass of 1 mole of
any substance
From the previous example using C-12, we now
have a mathematical relationship between
mass and moles, which is:
Mass of substance (g)
Number of moles (n) =
Molar mass (MM) (g/mol)
Example: How many moles are in 25 g of CO?
n CO = 25g CO/(12+16)g/mol
= 0.89 moles
We can also convert between moles and the
number of atoms or molecules using
Avogadro’s number
Number of atoms/molecules = moles (n) x NA
Example: How many atoms are there in a copper pipe that
weighs 2.56g?
n = 2.56/63.6 = 0.0403 moles
number of Cu atoms = 0.0403 X 6.022X1023
= 2.43X1022
We can now use the mole concept to determine
how much product to expect in a chemical
reaction. Take the following example:
2Fe2O3(s) + 3C(s)  4Fe(l) + 3CO2(g)
The coefficients in front of each species provide us with useful ratios that
we can use to calculate expected masses of products in a chemical
reaction. We previously said that these were ratios based on the
numbers of atoms. However, with Avogadro’s number, we can now say
that these are molar ratios.
We say, 2 moles of iron (III) oxide react with 3 moles of carbon to produce
4 moles of iron and 3 moles of carbon dioxide gas.
2Fe2O3(s) + 3C(s)  4Fe(l) + 3CO2(g)
Example:
How many grams of iron will we expect if we react 12g of
iron (III) oxide as in the above reaction, assuming
neither reactant is in excess?
Convert to moles
12g Fe2O3 X
=
1 mol Fe2O3
159.8g Fe2O3
12 X 1 X 4 X 55.9
159.8 X 2 X 1
Convert to g of
unknown
Molar ratio
X
=
4 mol Fe
2 mol Fe2O3
8.4 g Fe
X
55.9g Fe
1 mol Fe
Molecular mass is the sum of the atomic masses
of the atoms in a molecular formula.
Example:
The molecular mass of sucrose (table sugar) C12H22011 is
calculated as:
M.W. = (12XAC) + (22XAH) + (11XAO)
M.W. = (12X12.0) + (22X1.01) + (11X16.0)
= 342.2
Formula mass is the sum of the atomic masses in a
compound which has no discreet molecules (e.g. ionic
compounds). These describe the ratios of the atoms
present (i.e. empirical formulas), but are calculated
the same way as molecular masses.
Example:
The formula mass of calcium phosphate Ca3(PO4)2 is
calculated as:
F.W. = (3XACa) + (2XAP) + (8XAO)
F.W. = (3X40.1) + (2X31.0) + (8X16.0)
= 310.3
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What is the symbol for relative atomic mass?
Write the relative atomic masses for O, Mg, S.
Calculate the molecular mass of ethanol C2H5OH.
Calculate the molar mass of copper(II) sulfate CuSO4.
State the value and symbols of Avogadro’s Number.
How many atoms are there in 3 moles of nitrogen
atoms? 3 moles of ammonia?
How many grams are there in 3 moles of nitrogen? 3
moles of ammonia?
How many moles are there in 48g of water?