STOICHIOMETRY
4.1 Stoichiometry.
OBJECTIVES:
• Use the symbols of the elements and write the formulae of simple
compounds
• Deduce the formula of a simple compound from the relative numbers
of atoms present
• Deduce the formula of a simple compound from a model or a
diagrammatic representation
Element symbols
• Each element is represented by its own unique symbol as seen on the
Periodic Table e.g. H is hydrogen.
• Where a symbol contains two letters, the first one is always in capital
letters and the other is small e.g. sodium is Na, not NA.
• Atoms combine together in fixed ratios that will give them full outer
shells of electrons.
• The chemical formula is what tells you the ratio of atoms.
• E.g. H2O is a compound containing 2 hydrogen atoms which combine
with 1 oxygen atom.
• The chemical formula can be deduced from the relative number of
atoms present.
• E.g. if a molecule contains 3 atoms of hydrogen and 1 atom of
nitrogen then the formula would be NH3.
• Diagrams or models can also be used to represent the chemical
formula.
• The ammonia molecule consists of a central nitrogen atom bonded to
3 hydrogen atoms
Chemical formulae
• The structural formula tells you the way in which the atoms in a
particular molecule are bonded. This can be done by either a diagram
(displayed formula) or written (simplified structural formula).
• The empirical formula tells you the simplest whole number ratio of
atoms in a compound.
• The molecular formula tells you the actual number of atoms of each
element in one molecule of the compound or element e.g. H2 has 2
hydrogen atoms, HCl has 1 hydrogen atom and 1 chlorine atom.
Example: Butane
Structural formula (condensed)
CH3CH2CH2CH3
Molecular formula
C4H10
Empirical formula
C2H5
Deducing formulae by combining power(VALENCY)
• The concept of valency is used to deduce the formulae of
compounds.
• The valency of an element is the number of electrons its atoms lose,
• gain or share, to form a compound.
• Valency or combing power tells you how many bonds an atom can
make with another atom.
• E.g. carbon is in Group IV so a single carbon atom can make 4 single
bonds or 2 double bonds.
The following valencies apply to elements in each
group:
We can use the combining power of each
atom to work out a formula.
• This is how to write the formula of a compound, using valencies:
1. Write down the valencies of the two elements.
2. Write down their symbols, in the same order as the elements in the
name.
3. Add numbers after the symbols if you need to, to balance the
valencies.
Example 1 What is the formula of hydrogen
sulfide?
1. Valencies: hydrogen, 1; sulfur (Group VI), 2
2. HS (valencies not balanced)
3. The formula is H2S (2x1 and 2, so the valencies are now balanced)
Example 2 What is the formula of aluminium
oxide?
1. Valencies: aluminium (Group III), 3; oxygen (Group VI), 2
2. AlO (valencies not balanced)
3. The formula is Al2O3 (2 x3 and 3x2, so the valencies are now
balanced)
Alternatively:
• Write the symbols in the same order as in the name of the
compound.
• Write down the valencies underneath each element.
• Swap the numbers around.
• Where posiible, cancel down the numbers to get the smallest whole
number ratio when necessary.
• Ref. Table pg 49 in text book
NOTE:
• You can not predict the valency of transition elements.
• The formulae of some compounds containing only non metals are not
simplified by cancelling down.
Eg. H2O2
• Some non metals form oxides with different formulae.
• Eg CO, CO2
SO2,SO3
NO,NO2,N2O
Deducing Formulae of Ionic Compounds.
• Objective:
• Determine the formula of an ionic compound from the charges on the
ions present.
• The formulae of these compounds can be calculated if you know the
charge on the ions.
• Below are some common ions and their charges:
• For ionic compounds you have to balance the charge of each part by
multiplying each ion until the sum of the charges = 0.
• Example: what is the formula of Aluminium sulfate?
• Write out the formulae of each ion, including their charges.
• Al3+ SO42• Balance the charges by multiplying them out: Al3+ x 2 = +6 and
SO42- x 3 = -6; so +6 – 6 = 0.
• So the formula is Al2(SO4)3
The names and formulae of compounds
• The names of compounds:
• Many compounds contain just two elements. If you know which
elements they are, you can usually name the compound.
• Just follow these rules:
When the compound contains a metal and a non-metal:
– the name of the metal is given first
– and then the name of the non-metal, but ending with -ide.
• Examples: sodium chloride, magnesium oxide, iron sulfide.
When the compound is made of two non-metals:
– if one is hydrogen, that is named first
– otherwise the one with the lower group number comes first
– and then the name of the other non-metal, ending with -ide.
• Examples: hydrogen chloride, carbon dioxide.
But some compounds have ‘everyday’ names that give you no clue
about:
• the elements in them. Water, methane, and ammonia are examples.
• You just have to remember their formulae!
Finding formulae from the structure of
compounds.
• Every compound has a formula as well as a name. The formula is
made up of the symbols for the elements, and often has numbers
too.
• The formula of a compound is related to its structure. For example:
Note the difference:
• In giant structures like sodium chloride and silicon dioxide, the
formula tells you the ratio of the ions or atoms in the compound.
• In a molecular compound, the formula tells you exactly how many
atom2s are bonded together in each molecule.