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Measuring amounts of
substance
Relative atomic mass
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The link between the mass of an molecule and the
number of atoms it contains is the relative atomic
mass (Ar) of the element. It allows chemists to
calculate chemical formulae
The relative atomic mass scale is used to compare
the masses of different atoms.
The reference that used is the carbon-12 isotope
(C12) which is assigned a relative atomic mass of
exactly 12 every other atom is measured against this
Ar
Ar have no units. Carbon atoms do not have a mass
of 12 ‘anythings’ they are just 12 times heavier
than hydrogen atoms (Ar =1)
Chemical quantities
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12g of carbon, 1g of hydrogen and 16g of oxygen all contain equal
numbers of atoms because these masses are in the same ratio as
the relative atomic mass’
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The amounts of these substance has a special significance it is
called a mole (or mol for short)
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A unit that measures amount of substance in such a way that
equal amount of elements contain equal amounts of atoms
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the mass of one mole is equal to the relative atomic mass in grams
therefore the molar mass of carbon is 12g mol-1
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6g of C contains 6g ÷ 12g mol-1 = 0.5 mol of C atoms
4g of 0 contains 4g ÷ 16g mol-1 = 0.25 mol of O atoms
3g of H contains 3g ÷ 1g mol-1 = 3 mol of H atoms
Mass in grams ÷ molar mass = amount in moles of atoms
Relative formula mass
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Relative formula mass’
have no units, with
compounds such as
methane the mass is
usually called the
relative molecular
mass, Mr
Use moles to deal with all substances,
compounds as well as elements
As we use Ar for atoms we can use relative
formula mass for compounds
Methane CH4
Relative formula mass = (1 x12) + (4x1)
=16
calcium nitrate, Ca (NO3)2
Relative formula mass = (1x40) + (2x14) +
(6x16)
=164
Formula units
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bits that make up atoms and compound are called
formula units – they match the formula of substance the
formula unit for most elements e.g. Cu - the formula unit
is the Cu atom
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Exceptions the formula unit for oxygen gas is the O2
molecule rather than the O atom – formula mass of
oxygen is twice the A r of oxygen
In covalent compounds the formula unit is the molecule
e.g. CH4
In ionic compounds, the formula unit is the group of ions.
In calcium nitrate, the formula unit is Ca2+ (NO3-)2 and
contains a group of three ions - no special name for the
ions so we just call them formula units
Moles of formula units
Mass in grams ÷ molar = amount in moles
mass
of formula units
e.g. 8g of methane
8g ÷ 16g = 0.5 moles of methane
The Avogadro constant
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The number of formula units in a mole ( atoms,
electrons) is a constant. It is called the Avagadro
constant – L
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The value is - 6.02 x 1023
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The number is so huge its difficult to comprehend.
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If you want to make sure you have the same number
of atoms of different elements you don’t have to
count them you just have to measure a mole of that
substance e.g. 1 mole of carbon is exactly 12g – the
molar mass
Chemical formulae
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Use moles when we work out chemical formulae
To find out the formula of magnesium oxide you would
burn a known mass of magnesium and find out what
mass of oxygen combined with it
Mass of Mg = 0.84g
Amount of Mg atoms
= 0.84g / 24g mol-1 =0.035 mol
mass of O = 0.56g
amount of O atoms
= 0.56/16g mol-1
= 0.035 mol
Ratio of moles of Mg:O in magnesium atoms = 1:1
Formula of magnesium oxide = MgO
Calculating the %age mass of elements in
a compound - methane
% mass of C = 75%
% mass of H = 25%
Mass of C in 100g
of methane = 75g
Mass of H in 100g
of methane = 25g
Amount of C atoms
=75g / 12 g mol-1 =6.25 mol
Amount of H atoms
=25/1g mol -1 = 25 mol
Ratio of moles of atoms C:H in methane = 1:4
Formula of methane = CH4
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In a methane molecule there is a central carbon atom
surrounded by four hydrogen atoms. The simple ratio of atoms
from which it is formed is the same as in the formula of the
molecules
But it isn’t the case for all substances. Ethane molecules contain
eight atoms: two carbon atoms and six hydrogen atoms.
The molecular formula (tells us the actual numbers of
different types of atoms) of ethane is C2H6, but the simplest
ratio for the moles of atoms of C:H is 1:3. so a calculation from
percentage masses would lead you to a formula CH3. this formula
is the empirical formula- the ratio of the numbers of
different types of atoms in the substance
Some definitions
Empirical formula of a substance tells you the ratio of
the numbers of different types of atom in the substance
Molecular formula tells you the actual numbers of
different types of atom
The relative atomic mass of an element is the mass
of its atom relative to C12 = 12
The relative formula mass ( or relative molecular mass) of
a substance is the mass of its formula unit relative to C12
= 12
A mole of a substance is the amount of substance which contains
as many formula units (atoms, molecules, groups of ions, etc) as
there are atoms in 12g of C12 - 6.02 x 10 23
The molar mass of a substance is the mass of substance which
contains 1 mol