Stoichiochemistry

Question 1

Use the symbols of the elements and write the formulae of simple
compounds

Answer 1

● Examples of symbols of the elements: Sodium (Na), Chlorine (Cl) etc… (how they
are shown in the periodic table)
● Example of a formula of a simple compound: Sodium chloride (NaCl).

Question 2

(Extended only) Determine the formula of an ionic compound from the
charges on the ions present

Answer 2

To find the formula of simple compounds:
o work out what charge ion each element should form
o balance out the charges (so they equal zero) by increasing the number of
certain elements
o write the formula, using little numbers e.g. 2
to show if you have more
than one of a certain element
● E.g. Magnesium chloride :
o Magnesium forms a Mg2+ ion and chlorine forms a Clion
o Therefore, to balance the charges, you need to have 2 x Clions and 1 x
Mg2+ ion
o MgCl2

Question 3

Deduce the formula of a simple compound from the relative numbers of
atoms present

Answer 3

● for example, if you had 1 atom of aluminium (Al) for every 3 chlorine atoms (Cl),
the formula of the compound would be AlCl3

Question 4

(Extended only) Construct equations with state symbols, including ionic
equations

Answer 4

● (g) means gas, (s) means solid, (l) means liquid, (aq) means aqueous
● Example to find an ionic equation:
o Start with the balanced chemical equation
▪ HCl(aq) + NaOH(aq) → NaCl (aq) + H2O (l)
o Split up (aq) compounds into its ions
▪ H
+ + Cl- + Na+ + OH- → Na+ + Cl- + H2O
o Cancel out the ions that are the same on either side – known as
‘spectator ions’, leaving the ionic equation
▪ H
+ + OH- → H2O

Question 5

Deduce the formula of a simple compound from a model or a diagrammatic
representation
● work out how many of each different element you have in the compound and
then write the formula (remember to use smaller letters 3
to demonstrate how
many of each element you have)