Formulae Notes

Cambridge IGCSE Chemistry: Revision notes

Key facts

The molecular formula shows the actual atoms in a molecule; the empirical formula shows the simplest whole-number ratio.
For ionic compounds, total positive charge = total negative charge.
Equations must be balanced; only change the numbers in front of formulae.
State symbols: (s), (l), (g), (aq).
Ionic equations leave out spectator ions.

Molecular and empirical formulae

The empirical formula is the simplest ratio; the molecular formula is the actual number of each atom.

Hydrogen peroxide is $\ce{H2O2}$ (molecular) and $\ce{HO}$ (empirical). Glucose is $\ce{C6H12O6}$ and $\ce{CH2O}$ . The molecular formula is a whole-number multiple of the empirical formula, found using the relative molecular mass ( $M_r$ ).

1
Molecular formula
C₆H₁₂O₆
2
Divide by the highest common factor
6
3
Empirical formula
CH₂O

From molecular to empirical formula.

Hydrogen peroxide Glucose Molecular formula H₂O₂ C₆H₁₂O₆ Empirical formula HO CH₂O

	Hydrogen peroxide	Glucose
Molecular formula	H₂O₂	C₆H₁₂O₆
Empirical formula	HO	CH₂O

Hydrogen peroxide

Molecular formula:: H₂O₂
Empirical formula:: HO

Glucose

Molecular formula:: C₆H₁₂O₆
Empirical formula:: CH₂O

Worked example

A compound has empirical formula $\ce{CH}$ and relative molecular mass 78. Find its molecular formula.

1
Find the $M_r$ of the empirical formula $\ce{CH}$ : $12 + 1 = 13$ .
2
Divide: $78 \div 13 = 6$ .
3
Multiply every number in the empirical formula by 6.

What is the empirical formula of C₆H₁₂O₆?

Deducing a formula

Count the atoms, write the ratio, simplify; for ions, balance the charges.

Count each element, write the ratio, simplify by the highest common factor, and write it as subscripts. For ionic compounds, the total positive charge must equal the total negative charge.

Ca²⁺

2,8,8

Cl⁻

2,8,8

Cl⁻

2,8,8

CaCl₂: one Ca²⁺ (2+) needs two Cl⁻ (1− each) to balance the charges.

Ions Working Formula Magnesium oxide Mg²⁺, O²⁻ 2+ and 2- balance, so 1 of each MgO Calcium chloride Ca²⁺, Cl⁻ One 2+ needs two 1- CaCl₂ Aluminium oxide Al³⁺, O²⁻ Two 3+ (6+) balance three 2- (6-) Al₂O₃

	Ions	Working	Formula
Magnesium oxide	Mg²⁺, O²⁻	2+ and 2− balance, so 1 of each	MgO
Calcium chloride	Ca²⁺, Cl⁻	One 2+ needs two 1−	CaCl₂
Aluminium oxide	Al³⁺, O²⁻	Two 3+ (6+) balance three 2− (6−)	Al₂O₃

Ions

Magnesium oxide:: Mg²⁺, O²⁻
Calcium chloride:: Ca²⁺, Cl⁻
Aluminium oxide:: Al³⁺, O²⁻

Working

Magnesium oxide:: 2+ and 2− balance, so 1 of each
Calcium chloride:: One 2+ needs two 1−
Aluminium oxide:: Two 3+ (6+) balance three 2− (6−)

Formula

Magnesium oxide:: MgO
Calcium chloride:: CaCl₂
Aluminium oxide:: Al₂O₃

Worked example

A model shows 4 sodium atoms, 2 sulfur atoms and 8 oxygen atoms. Write the formula.

1
The ratio Na : S : O is 4 : 2 : 8.
2
Divide by 2 to give 2 : 1 : 4.

What is the formula of calcium chloride?

Word and symbol equations

Write the formulae, balance the atoms with numbers in front, then add state symbols.

Word equations give names: reactants $\rightarrow$ products. Symbol equations give formulae and must be balanced, with each element having the same number of atoms on both sides. State symbols follow each formula: (s) solid, (l) liquid, (g) gas, (aq) aqueous.

1
Write
the unbalanced equation with correct formulae
2
Count
the atoms of each element on both sides
3
Balance
by adding numbers in front of formulae
4
Check
all elements are balanced
5
Add
state symbols

Balancing a symbol equation

Worked example

Copper reacts with oxygen to form copper oxide. Write the balanced symbol equation with state symbols.

1
Word equation: copper + oxygen $\rightarrow$ copper oxide.
2
Unbalanced: $\ce{Cu + O2 -> CuO}$ .
3
There are 2 O on the left and 1 on the right, so put 2 in front of $\ce{CuO}$ , then 2 in front of $\ce{Cu}$ .

What does (aq) mean?

Ionic equations

An ionic equation shows only the particles that react, leaving out spectator ions.

Spectator ions are the same on both sides. Write the full equation, split soluble ionic compounds into ions, cancel the spectators, and write the net ionic equation.

1
Full equation
with state symbols
2
Full ionic
soluble ionic compounds as separate ions
3
Spectators
ions unchanged on both sides
4
Cancel
remove them
5
Net ionic equation
what remains

Writing a net ionic equation

Hydrochloric acid with sodium hydroxide:

Full: $\ce{HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l)}$
Full ionic: $\ce{H+(aq) + Cl-(aq) + Na+(aq) + OH-(aq) -> Na+(aq) + Cl-(aq) + H2O(l)}$
Net ionic: $\ce{H+(aq) + OH-(aq) -> H2O(l)}$

$\ce{Na+}$ and $\ce{Cl-}$ are the spectator ions.

What are spectator ions?

Deducing equations

Identify the reaction type, write correct formulae, then balance and add state symbols.

Use the names, states, reaction type or data given. Combustion gives carbon dioxide and water if the fuel contains C and H; oxidation gives an oxide; acid + base gives salt + water; displacement swaps a less reactive element out; decomposition gives simpler substances.

1
Identify
the reactants and the products
2
Formulae
write the correct formula for each substance
3
Unbalanced
write the equation
4
Balance
by counting the atoms of each element
5
States
add state symbols from the conditions

Deducing an equation

Worked example

Deduce the equation for the reaction between sodium carbonate solution and dilute hydrochloric acid.

1
Reactants: $\ce{Na2CO3(aq)}$ and $\ce{HCl(aq)}$ . Products: $\ce{NaCl}$ , $\ce{CO2}$ and $\ce{H2O}$ .
2
Unbalanced: $\ce{Na2CO3 + HCl -> NaCl + CO2 + H2O}$ .
3
There are 2 Na on the left, so put 2 in front of $\ce{NaCl}$ . That makes 2 Cl, so put 2 in front of $\ce{HCl}$ .

Burning magnesium in oxygen gives which equation?

Try an exam question

Write the balanced symbol equation, with state symbols, for the reaction of magnesium with oxygen to form magnesium oxide.

[3 marks]

That's the notes covered.

Carry on to the next subtopic.