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Chemistry module 2
Chemistry module 2.2
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Revision Notes
, Topic Questions, Past Papers
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OCR A Level
Chemistry
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Contents
2.2.1
Amount of Substance
2.2.2
Determining Formulae
2.2.3
Reaction Calculations
2.2.4
The Ideal Gas Equation
2.2.5
Percentage Yield & Atom Economy
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Your
notes
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Amount of
substance
The name given when counting the
number
of
particles
in a substance
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Amount of
substance
Often
seen in calculations using the letter / symbol n
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Units for amount of substance
moles
/
mol
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Avogadro constant, N
The number of particles equivalent to the relative atomic, molecular or
formula mass
of a substance
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The
Avogadro
constant applies to atoms, molecules, ions and
electrons
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Value of N
6.02
x 10^
23
g mol^-1
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Mass of a substance with this number of particles
A
mole
(
mol
)
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Molar mass
The mass of
substance
that contains the same number of fundamental units as exactly
12.00
g of carbon-12
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Relationship between amount of substance, n, mass, m, and molar mass, M
n =
mass
, m /
Molar mass
, M
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Molar gas volume
The volume occupied by one
mole
of any
gas
, at room temperature and pressure
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Molar volume
Equal to
24
dm^3
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One
mole
of any element is equal to the relative
atomic mass
of the element, in grams
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One mole of carbon is
6.02
x 10^23 atoms of carbon with a mass of
12.0
g
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One mole of water is
6.02
x 10^23 molecules of water with a mass of
18.0
g
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Calculating molar mass and molar gas volume
1. Calculate the molar mass of: a.
Carbon dioxide
, CO2 b.
Magnesium nitrate
, Mg(NO3)2
2. Calculate the number of
moles
of each
gas
: a. 36.0 dm^3 of carbon monoxide, CO b. 9.6 dm^3 of chlorine, Cl2
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Molecular formula
Shows the
number
and
type
of each atom in a molecule
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Empirical formula
Shows the simplest whole number
ratio
of the elements present in
one
molecule of the compound
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The
empirical
formula of ethanoic acid is
CH2O
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The
molecular
formula of ethanoic acid is
C2H4O2
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Deducing molecular & empirical formulae
Deduce the
molecular
and
empirical formula
of the following compounds:
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Calculating empirical formula from mass
Determine the empirical formula of a compound that contains
10
g of hydrogen and
80
g of oxygen.
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Calculating empirical formula from percentage composition
Determine the empirical formula of a compound that contains
85.7%
carbon and
14.3%
hydrogen.
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Calculating
molecular formula
The empirical formula of
X
is CH2 and the relative molecular mass of
X
is 180.2. What is the molecular formula of X?
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Water of crystallisation
When some compounds can form
crystals
which have
water
as part of their structure
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Hydrated
compound
A compound that contains
water
of
crystallisation
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Anhydrous
compound
A compound which doesn't contain
water
of
crystallisation
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Cobalt(II) chloride
can be
hydrated
by six or two water molecules: CoCl2·6H2O or CoCl2·2H2O
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Calculating water of crystallisation
10.0
g of hydrated copper sulfate are heated to a constant mass of 5.59 g. Calculate the formula of the
original
hydrated copper sulfate.
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Reacting
masses
The masses of reactants are
useful
to determine how much of the reactants exactly react with each other to prevent
waste
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Mass calculation using moles
Calculate the mass of
magnesium oxide
that can be made by completely burning 6 g of magnesium in
oxygen.
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Moles
The amount of a substance, measured in
mol
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Molar mass
The mass of
one
mole of a substance, measured in
g mol−1
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Formula unit
The smallest
repeating
unit of a compound
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One
mole
of CaF2 contains one
mole
of CaF2 formula units, but one mole of Ca and two moles of F ions
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