Topic 1.3 - Formulae, equations and amounts of substance
Lesson 1 - The basics
a. demonstrate an understanding of the terms atom, element, ion, molecule,
compound, empirical and molecular formulae
b. write balanced equations (full and ionic) for simple reactions, including the
use of state symbols
c. demonstrate an understanding of the terms relative atomic mass, amount
of substance, molar mass and parts per million (ppm), eg gases in the
atmosphere, exhausts, water pollution
Connector:
1. Define an atom, a compound and an ion.
2. Draw a lithium atom ( At. No. 3; At. Mass 7)
3. List the THREE main sub-atomic particles, and give
their relative masses and electrical charges.
Crowe 2011
An atom is the smallest, electrically neutral, particle of
an element that can take part in a chemical change.
A molecule is the smallest, electrically neutral,
particle of an element or compound that can exist on
its own.
An ion is an atom, or group of atoms, which carries an
electric charge.
Lithium Atom
A Lithium has 3 protons and 4 neutrons inside the nucleus with
3 electrons orbiting around the nucleus.
Write the word equation and then the balanced equation,
including state symbols for the following reactions:
1. Hydrogen and oxygen combining to form water.
2. Calcium carbonate decomposing into calcium oxide
and carbon dioxide.
3. Hydrochloric acid reacting with magnesium to produce
hydrogen gas and a solution of magnesium chloride.
4. Sulphuric acid being neutralised by sodium hydroxide
to give a solution of sodium sulphate.
5. Zinc oxide reacting with sulphuric acid to give a
solution of zinc sulphate.
6. Aluminium combining with oxygen to form its oxide.
7. Methane under going complete combustion.
Relative atomic mass & molar mass
Explain these terms.
RAMS
&
MOLES
Relative atomic mass
The definition of Relative Atomic Mass (Ar):
The mass of a single atom on a scale on which the mass of an atom
of carbon—12 has a mass of 12 atomic mass units.
12 hydrogen atoms have the
same mass as 1 carbon atom
The relative atomic mass does not have units.
Relative Atomic Mass of elements
The atoms of each element have a different mass.
Carbon is given a relative atomic mass (RAM) of 12.
The RAM of other atoms compares them with carbon.
Eg. Hydrogen has a mass of only one twelfth that of carbon and so has a RAM of 1.
Below are the RAMs of some other elements.
Element
Symbol
Times as heavy as carbon
R.A.M
Helium
He
one third
4
Beryllium
Be
three quarters
9
Molybdenum
Mo
Eight
96
Krypton
Kr
Seven
84
Oxygen
O
One and one third
16
Silver
Ag
Nine
108
Calcium
Ca
Three and one third
40
Molar mass or Moles
The mole is the formula mass in g’s of an atom,
molecule, or compound.
Example: Water
H2O = (2xH)+(O) = (2x1)+16 = 18
So water has a molar mass of 18g
or
1 mole of water weighs 18g
Formula Mass
Calculate the formula mass of the compounds below.
(N=14; H=1; Na=23; O=16; Mg=24; Ca=40)
Substance
Formula
Formula Mass
Ammonia
NH3
14 + (3x1)=17
Sodium oxide
Na2O
(2x23) + 16 =62
Magnesium hydroxide
Mg(OH)2
24+ 2(16+1)=58
Calcium nitrate
Ca(NO3)2
40+ 2(14+(3x16))=164
What is the relationship between molar mass and formula mass?
Molar mass if the formula mass in g
Calculating Moles
The number of moles present in a
weighed sample can be calculated using
this equation:
Number of moles = mass
R.A.M.
Similarly
mass = moles x R.A.M.
1. How many moles are there in
2. What is the mass of
a. 4.4g of carbon dioxide?
a. 3 moles of hydrogen gas?
b. 80g of methane?
b. 0.5 mole of copper(II) oxide?
c. 14.2g of chlorine?
c. 4 moles of sodium chloride?
d. 128g of sulphur?
d. 0.3 mole of ethene?
e. 1kg of calcium carbonate?
e. 0.25 mole helium?
Percentage Composition
% Z = (Number of atoms of Z) x (atomic Mass of Z)
Formula Mass of the compound
Calculate the percentage of oxygen in carbon dioxide
CO2
12 +(2x16)=44
2 x 16 / 44 = 72.7%
parts per million (ppm)
This is a way of expressing very dilute concentrations of
substances e.g gases in the atmosphere or in exhaust fumes,
pollutants in a river, etc.
Just as per cent means out of a hundred,
so
parts per million, or ppm, means out of a million.
Convert 0.025% into ppm
0.025
=
100
?
1000000
1000000 x 0.025 = ? = 250ppm
100
Convert 0.19g/litre into ppm
0.19 =
1000
?
1000000
1000000 x 0.19 = ? = 190ppm
1000
Past paper questions
In 2006, the concentration of carbon dioxide in the atmosphere was
382 ppm. This is equivalent to
382
1000000
=
x
100
A halogenoalkane, bromomethane, CH3Br, is a toxic gas used to
protect plants against insects. Health and Safety advice states that
concentrations above 5 parts per million (ppm) by volume of this gas
are harmful.
A research laboratory contains 2.5 × 105 dm3 of air.
Calculate the maximum volume of bromomethane, in dm3, allowed in
the laboratory to comply with the advice given.
Jan 09
Exercise 1: Calculation of the Molar Mass of compounds
MFE Booklet pp 6-8