Transcript Relative Atomic Mass and the Mole

Relative Atomic Mass
and the Mole
Chapter 4 of Hienemann Textbook
Relative Isotopic Mass
Knowing the mass of an individual atom is not very useful
because we usually work with large quantities of atoms.
The International Union of Physics and Chemistry decided that
the relative atomic weights of elements would be calculated
relative to the most common isotope of carbon: 12C.
Element
Hydrogen
Carbon
Isotopes
Relative Isotopic
Mass
Abundance (%)
1H
1.008
99.986
2H
2.014
0.014
3H
3.016
0.0001
12C
12 EXACTLY
98.888
13C
13.003
1.112
14C
14.003
Approx. 10-10
Relative Atomic Mass
A naturally occurring sample of an element
contains the same isotopes in the same
proportions, regardless of the source.
Therefore we use an average of the relative
isotopic masses of the isotopes that make
up an element  the relative atomic mass.
Relative atomic mass has the symbol Ar
Example
Imagine taking 100 atoms from a sample of
chlorine.
There will be:
• 75.80 atoms of 35Cl
• 24.20 atoms of 37Cl
Average relative mass of one chlorine atom:
= 34.969 x 75.8 + 36.966 x 24.20
100
= 35.452
The relative atomic mass of chlorine is 35.45
Ar (Cl) = 35.45
Relative Molecular Mass
The relative molecular mass (Mr) of a compound is the
mass of one molecule of that substance relative to the
mass of a 12C atom.
e.g. Oxygen
Mr(O2) = 2 x Ar (O)
= 2 x 16.0
= 32.0
Have a go!
Find the relative molecular mass of CO2
Mr(CO2) = Ar (C) + 2 x Ar (0)
= 12.0 + 2 x 16.0
= 44.0
What is a mole?
Since atoms are so small it is impossible to
measure out how many individual atoms of a
particular element is needed in reactions.
Therefore, a unit called the mole was
established which allows us to measure out
an exact number of atoms.
One mole contains:
6.02 x 1023 entities
(entities can be atoms, molecules etc.)
• One mole of sodium ions contains 6.02 x 1023
sodium ions
• One mole of water contains 6.02 x 1023 water
molecules
• One mole of doughnuts
contains 6.02 x 1023
doughnuts
The weight of a mole depends on what substance is
being measured.
e.g. 6.02 x 1023 peas weighs less than 6.02 x 1023
watermelons
The weight of 1 mole of atoms of an element is
defined as the relative atomic mass of that
element in grams.
e.g. What is the weight of 1 mole of Magnesium?
M(Mg) = 24g mol-1
Atomic mass
M means molar mass
(mass of 1 mole)
mol-1 means per mole
Give it a go!
How much will 1 mole of Helium weigh?
M(He) = 4g mol-1
Molar Mass
One mole of any substance (an element or a
compound) will have a mass in grams equal to
the sum of the relative atomic masses of all the
atoms in the formula.
This is called the molar mass (M).
e.g. one mole of chlorine gas (Cl2) weighs:
M(Cl2) = 2 x Ar(Cl)
= 2 x 35.5
= 71.0g
Give it a go!
Calculate the Molar mass of hydrogen
chloride (HCl).
M(HCl) = Ar (H) + Ar (Cl)
= 1.00 + 35.5
= 36.5g
Solving Problems with the Mole
The number of moles
of a substance =
Mass of substance
Molar mass of the substance
Using symbols this can be written as……
n= m
n is the symbol for
number of moles
M
m is the symbol for
mass (of the sample
M is the symbol for
molar mass
An easy way to remember this
equation…..
m
n
Cover up the corner of the
triangle that symbolises the
value you are looking for.
M
e.g. if you need to calculate the mass of a sample and you know the number of
moles and the molar mass of the sample
Cover the m
 equation: m = n x M
Give it a go! What is the equation if you’re looking for Molar mass?
M = m/n
Give it a go!
How much does 0.024 mole of Chlorine gas (Cl2)
weigh?
1. Work out what you’re calculating: mass
2. Equation to use: m = n x M
3. Substitute in the values that you have been given:
m = 0.024 x (2 x Ar (Cl))
m = 0.024 x (2 x 35)
m = 0.024 x 70
m = 1.68
4. Remember the units: 1.68g