Ions to Formulae and Equations
Ions to formulae
In the correct formula of a compound the charges on the ion must cancel out. Using the ion table attached
and the set of rule cards below the excercises, you can turn the name of a compound into its formula.
Start with the name :- sodium
write in the ions :balance the charges :write the formula :-
chloride
iron III
oxide
lithium sulphate
Cheats Method :- sodium
write in the ions :swap the numbers :cancel down :-
chloride
iron III
oxide
lithium sulphate
Use either method to find the formulae of the following compounds.
Exercise 1
a.
b.
c.
d.
e.
f.
Sodium fluoride
Lithium iodide
Magnesium sulphide
Calcium oxide
Aluminium nitride
Iron (III) phosphide
g.
h.
i.
j.
k.
l.
Magnesium chloride
Sodium sulphide
Aluminium iodide
Lithium nitride
Magnesium nitride
Aluminium oxide
g.
h.
i.
j.
k.
l.
Iron(II) sulphate
Hydrogen chloride
Silver nitrate
Sodium hydrogencarbonate
Lead (II) carbonate
Ammonium hydroxide
Exercise 2
a.
b.
c.
d.
e.
f.
Potassium nitrate
Sodium hydroxide
Magnesium sulphate
Calcium carbonate
Aluminium phosphate
Ammonium chloride
In the next exercise you may need to use brackets in the formulae. We use brackets around polyatomic ions (sulphate,
nitrate, ammonium) whenever we need two or more of them ;
e.g. sodium hydroxide, NaOH
copper hydroxide, Cu(OH)2
aluminium hydroxide, Al(OH)3
Exercise 3
a.
b.
c.
d.
e.
f.
Copper (II) nitrate
Sodium sulphate
Calcium hydroxide
Sodium hydrogensulphate
Aluminium nitrate
Potassium hydrogen phosphate
g.
h.
i.
j.
k.
l.
Iron (III) sulphate
Copper (II) ethanoate
Ammonium carbonate
Calcium phosphate
Iron (III) carbonate
Ammonium sulphate
Ions to Formulae and Equations
Now balance some simple equations.
Write the word equation and the write in the formulae underneath it
Sodium hydroxide + hydrochloric acid  sodium chloride + water
NaOH + HCl  NaCl + H2O
In a balanced equation there will be the same number of atoms of each element on both sides of the arrow. In this
example there is one sodium atom in the sodium hydroxide and one in the sodium chloride. There is one atom of
hydrogen on each side also. Count the hydrogens and the chlorine.
Exercise 4
1. Calcium carbonate  calcium oxide + carbon dioxide
2. Hydrochloric acid + sodium hydroxide sodium chloride + water
3. Magnesium + sulphuric acid  magnesium sulphate + hydrogen gas
4. Copper sulphate + zinc  zinc sulphate + copper
5. Iron II oxide + calcium  calcium oxide + iron
In more difficult equations you need to multiply some of the formulae to balance the elements;
Copper + oxygen gas  copper oxide
Cu + O2  CuO
Start with what changes; there are 2 O’s in O2 and only one in CuO. To balance it you need 2 O’s in the products. Don’t
be tempted to write CuO2, you have found the formula and you can’t change it. Instead double the CuO
Cu + O2 
2CuO
Now you have balanced the oxygen, you have 2 O’s in O2 and in the 2CuO
But you now have 2Cu’s in the products but only one in the reactants, so double the Cu
2Cu +
O2 
2CuO
Quickly double check each element, all done.
Ions to Formulae and Equations
Try these harder ones; hydrogen gas is H2, oxygen gas is O2
Exercise 5
1. Magnesium + oxygen gas  magnesium oxide
2. Aluminium + oxygen gas  aluminium oxide
3. Zinc + hydrochloric acid  zinc chloride + hydrogen gas
4. Copper oxide + nitric acid  copper nitrate + water
5. Calcium carbonate + hydrochloric acid  calcium chloride + water + carbon dioxide gas
6. Sodium + oxygen gas  sodium oxide
7. Sodium hydroxide + sulphuric acid  sodium sulphate + water
8. Sodium + water  sodium hydroxide + hydrogen gas
9. Aluminium + Iron (III) oxide  aluminium oxide + iron
10. Magnesium nitrate  magnesium oxide + oxygen gas + nitrogen dioxide (NO2)
The last one is difficult, if you can’t balance it first time through, try starting with two magnesium nitrates.
Ions to Formulae and Equations
Answers
Exercise 1
a.
b.
c.
d.
e.
f.
Sodium fluoride
Lithium iodide
Magnesium sulphide
Calcium oxide
Aluminium nitride
Iron (III) phosphide
NaF
LiI
MgS
CaO
AlN
FeP
g.
h.
i.
j.
k.
l.
Magnesium chloride
Sodium sulphide
Aluminium iodide
Lithium nitride
Magnesium nitride
Aluminium oxide
KNO3
NaOH
MgSO4
CaCO3
AlPO4
NH4Cl
g.
h.
i.
j.
k.
l.
Iron(II) sulphate
Hydrogen chloride
Silver nitrate
Sodium hydrogencarbonate
Lead (II) carbonate
Ammonium hydroxide
MgCl2
Na2S
AlI3
Li3N
Mg3N2
Al2O3
Exercise 2
a.
b.
c.
d.
e.
f.
Potassium nitrate
Sodium hydroxide
Magnesium sulphate
Calcium carbonate
Aluminium phosphate
Ammonium chloride
FeSO4
HCl
AgNO3
NaHCO3
PbCO3
NH4OH
In the next exercise you may need to use brackets in the formulae. We use brackets around polyatomic ions (sulphate,
nitrate, ammonium) whenever we need two or more of them ;
e.g. sodium hydroxide, NaOH
copper hydroxide, Cu(OH)2
aluminium hydroxide, Al(OH)3
Exercise 3
a.
b.
c.
d.
e.
f.
Copper (II) nitrate
Sodium sulphate
Calcium hydroxide
Sodium hydrogensulphate
Aluminium nitrate
Potassium hydrogenphosphate
Cu(NO3)2
Na2SO4
Ca(OH)2
NaHSO4
Al(NO3)3
K2HPO4
g.
h.
i.
j.
k.
l.
Iron (III) sulphate
Copper (II)ethanoate
Ammonium carbonate
Calcium phosphate
Iron (III) carbonate
Ammonium sulphate
Fe2(SO4)3
Cu(CH3COO)2
(NH4)2CO3
Ca3(PO4)2
Fe2(CO3)3
(NH4)2SO4
Ions to Formulae and Equations
Exercise 4
1. Calcium carbonate  calcium oxide + carbon dioxide
CaCO3
 CaO + CO2
2. Hydrochloric acid + sodium hydroxide sodium chloride + water
HCl + NaOH
 NaCl + H2O
3. Magnesium + sulphuric acid  magnesium sulphate + hydrogen gas
Mg + H2SO4
 MgSO4 + H2
4. Copper sulphate + zinc  zinc sulphate + copper
CuSO4 + Zn
 ZnSO4 + Cu
5. Iron II oxide + calcium  calcium oxide + iron
FeO + Ca
 CaO + Fe
Exercise 5
1. Magnesium + oxygen gas  magnesium oxide
Mg + O2
 MgO
2. Aluminium + oxygen gas  aluminium oxide
4Al + 3O2
 2Al2O3
3. Zinc + hydrochloric acid  zinc chloride + hydrogen gas
Zn + 2HCl
 ZnCl2 + H2
4. Copper oxide + nitric acid  copper nitrate + water
CuO + 2HNO3 Cu(NO3)2 + H2O
5. Calcium carbonate + hydrochloric acid  calcium chloride + water + carbon dioxide gas
CaCO3 + 2HCl CaCl2 + H2O + CO2
6. Sodium + oxygen gas  sodium oxide
4Na + O2
 2Na2O
7. Sodium hydroxide + sulphuric acid  sodium sulphate + water
2NaOH + H2SO4  Na2SO4 + 2H2O
8. Sodium + water  sodium hydroxide + hydrogen gas
2Na + 2H2O
 2NaOH + H2
9. Aluminium + Iron (III) oxide  aluminium oxide + iron
2Al + Fe2O3
 Al2O3 + 2Fe
10. Magnesium nitrate  magnesium oxide + oxygen gas + nitrogen dioxide (NO2)
2Mg(NO3)2  2MgO + O2 + 4NO2 OR
Mg(NO3)2
 MgO + 1/2O2 + 2NO2
Ions to Formulae and Equations
Rule 1
Write the symbols for the
cation and the anion
e.g.
Na
&
Cl
Ions to Formulae and Equations
RULE 2
Determine the charge on the
cation and anion from their
position on the periodic table
+
Na
&
-
Cl
Ions to Formulae and Equations
Deducing the charge on the ion from the
periodic table; how does it work?

Group I has only one electron in its outer shell, so when it loses that
electron it will then have a +1 charge.

Groups II and III lose two and three electrons in their outer shells
respectively to become charged +2 and +3.

Group IV can go either way. It can either lose or gain four electrons. It
rarely forms ions, though.

Group V with its five electrons in its outer shell is when things change.
Group V will gain three electrons to have a -3 charge.

Group VI gains two electrons to have a -2 charge.

Group VII has seven electrons in its outer shell, so it gains one electron to
have a -1 charge
Ions to Formulae and Equations
RULE 3
If the cation has a Roman numeral
after it, that is the charge on that
cation. Cations receive Roman
numerals when they can take more
than one ionic form, e.g. Iron (III)
Oxide
Ions to Formulae and Equations
RULE 4
Write the two symbols together, and
determine how to make the
compound neutral by finding the
lowest common multiple of the
charges on each ion. Then figure out
how atoms of each element are
needed to make that charge e.g. NaCl
Ions to Formulae and Equations
Let's try working out Sodium Oxide
1.
Write Na and O.
2.
Put in the charges so write Na+ and O2-.
3.
Na+O2-, the lowest common multiple is 2.
To get a charge of 2 on sodium (which has a
+1 charge), you need to multiply it by 2 (1 ×
2 = 2). So you will have two atoms of
sodium. To get a charge of 2 on the oxygen,
you multiply it by 1 (2 × 1 = 2). So, just one
O. Therefore we write Na2O
Try working out Iron (III) Oxide
Ions to Formulae and Equations
Polyatomic ions
Writing formulas for polyatomic ionic
compounds isn't hard when you know the
formula for the polyatomic ion. There are many
tables available that show the formula for
common polyatomic ions and the charge on
them. Over time, you will easily memorise the
ones you use most. Use the same steps as when
you name simple ionic compounds, but treat the
polyatomic ion as one unit.
Ions to Formulae and Equations
Let's try an example. Iron (III) Chromate.
1.
Write Fe and CrO4.
2.
Put in the charges, so write Fe3+ and CrO42-
3.
Balance the charges and you end up with
Fe2(CrO4)3 .
The lowest common multiple of the +3 charge on the
iron and the -2 charge on the chromate is 6. So, you
need two iron ions (3 × 2 = 6) and three chromate ions
(2 × 3 = 6) to make a neutral compound. Fe2(CrO4)3. The
three outside the brackets indicates that everything
inside those brackets is multiplied by that number.
Try working out Ammonium Phosphate.
Ions to Formulae and Equations
Writing & balancing equations
A chemical equation describes what happens in a chemical
reaction. The equation identifies the reactants (starting materials)
and products (resulting substance), their formulae, phases (solid,
liquid, gas), and the amount of each substance.
o Write the unbalanced equation.
o Chemical formulas of reactants are listed on the left-hand side of the
equation.
o Products are listed on the right-hand side of the equation.
o Reactants and products are separated by putting an arrow between
them to show the direction of the reaction.
o Get the same number of atoms of every element on each side of the
equation.
o Once one element is balanced, proceed to balance another, and
another, until all elements are balanced.
o Balance chemical formulae by placing coefficients in front of them.
Do not add subscripts, because this will change the formulae.
Ions to Formulae and Equations
o
Example
Look at the equation below and see which elements are not
balanced. In this case, there are two oxygen atoms on the lefthand side of the equation and only one on the right-hand side.
Correct this by putting a coefficient of 2 in front of water:
SnO2 + H2 → Sn + H2O
This puts the hydrogen atoms out of balance. Now there are
two hydrogen atoms on the left and four hydrogen atoms on
the right. To get four hydrogen atoms on the left, add a
coefficient of 2 for the hydrogen gas. Remember, coefficients
are multipliers, so if we write 2 H2O it denotes 2x2=4 hydrogen
atoms and 2x1=2 oxygen atoms.
SnO2 + 2 H2 → Sn + 2 H2O
The equation is now balanced. Be sure to double-check your
math! Each side of the equation has 1 atom of Sn, 2 atoms of O,
and 4 atoms of H.