Download Relative atomic and molecular mass

Relative atomic and
molecular mass
Including introduction to the mole
Objectives:
• Recap of the structure of the atom
• To understand the need for relative atomic mass
• How to find relative atomic mass on the periodic table
• To know how to calculate relative formula (molecular)
mass
• Checking balanced equations with relative formula masses
• Definition of the mole
• Calculation of molecular masses
Mass number tells
us how many
protons and
neutrons are in the
nucleus
23
Proton number
tells us how many
protons are in the
nucleus
Na
11
What does this tell us?
Recap…
Mass of a proton?
Neutron?
Electron?
Atomic Mass Units
The actual mass of a hydrogen atom is 1.7x10-24g
(that’s 0.0000000000000000000000017g!)
Far too small a number to easily get your head
around…
So – we use Relative Atomic Masses.
Relative Atomic Mass (Ar) tells us the mass of
an atom compared to 1 atom of carbon.
Carbon is given a Ar of 12 (as it has 6 protons
and 6 neutrons).
What is relative atomic mass?
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Where are RAM values found?
The values of relative atomic mass (RAM) are usually
given in a data book or found in the periodic table. So you
don’t have to work them out or remember them all!
When looking up relative atomic mass in the periodic table,
remember that it always the larger of the two numbers given.
What is the other number?
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The relative atomic mass is not always a whole number.
This is because it also takes into account the various isotopes
of that element and their relative abundance
Occurs naturally as two isotopes
Which isotope is in greater abundance naturally?
Identifying relative atomic mass
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How is relative formula mass calculated?
To find the relative formula mass of a compound, add up
the relative atomic masses of all the atoms in its formula.
Step 1: Write down the formula of the molecule.
Step 2: Find the RAM of each type of atom in the molecule.
Step 3: Multiply each RAM by the number of atoms of that
element and add these values together.
What is the relative formula mass of water?
Step 1: formula of water is H2O
Step 2: RAM values: hydrogen = 1, oxygen = 16
Step 3: relative formula mass = (2 × 1) + (1 × 16) = 18
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We can use relative atomic masses to work out
the relative formula mass (Mr) of a molecule or
compound.
E.g. NaCl
Ar of Na = 23
Ar of Cl = 35.5
So Mr of NaCl = 23 + 35.5 = 58.5
Calculate the relative formula mass of the following
compounds (showing your working!):
(a) Carbon monoxide CO
(b) Carbon dioxide
(c) Sulphur dioxide SO2
(d) Calcium carbonate CaCO3
(e) Sodium hydroxide NaOH
(f) Sulphuric acid H2SO4
(g) Hydrochloric acid HCl
(h) Copper sulphate CuSO4
(i) Magnesium chloride MgCl2
(j) Sodium carbonate Na2CO3
(k) Lead nitrate Pb(NO3)2
(l) Calcium hydroxide Ca(OH)2
In a balanced equation, the sum of the relative formula masses
of the reactants in the quantities shown is equal to the sum of
the relative formula masses of the products in the quantities
shown.
Check that this is true for:
2Mg + O2
2MgO
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
Just How Big is a Mole?
• Enough soft drink cans to cover the
surface of the earth to a depth of
over 200 miles.
• If you had Avogadro's number of
unpopped popcorn kernels, and
spread them across the United
States of America, the country would
be covered in popcorn to a depth of
over 9 miles.
• If we were able to count atoms at the
rate of 10 million per second, it
would take about 2 billion years to
count the atoms in one mole.
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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
What is the mass of one mole?
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Relative masses – true or false?
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