Monday, 5 January 2015

3.5 Chemical Formulae

Chemical Formulae
■ Chemical Formulae

To represent a chemical compound.

The elements present in the compound which are denoted by their symbols.

The relative numbers of each element present in the compound which are indicated by subscripts written after the symbols.
■ Example

Several covalent compounds and their chemical formulae.

Number of atoms of each element making up the compound.

Example of electron configuration and valence electrons.
Compound Chemical formula Number of each elements
Oxygen O2 2 oxygen atom
Chlorine Cl2 2 chlorine atom
Carbon dioxide CO2 1 carbon atom and 2 oxygen atom
Nitrogen dioxide NO2 1 nitrogen atom and 2 oxygen atom
Sulphuric Acid H2 SO4 2 hydrogen atom, 1 sulphur atom and 4 oxygen atom
Nitric Acid HNO3 1 hydrogen atom, 1 nitrogen atom and 3 oxygen atom
■ There are two types of chemical formula:

(a) Empirical formula.

(b) Molecular formula.

Empirical formula and Molecular formula
■ Empirical formula:

Definition: Empirical formula: Shows the simplest ratio of the atoms to the elements that combine to form the compound.
■ Molecular formula:

Definition: Molecular formula: Shows the actual number of atoms of the elements that combine to form the compound.

The molecular formula is the multiple of the empirical formula.
Molecular formula = (Empirical formula)n , where n is an integer.

Example :
Empirical formula of glucose, C6H12O6 = CH2O
[Divide the number of atom with 6 to get simplest ratio of atoms]
Molecular formula of glucose = C6H12O6
■ The following steps should be followed to obtain the empirical formula of a compound:

Step 1: Find the mass or percentage of elements in compound.

Step 2: Divide mass or percentage by relative atomic mass(RAM) to find number of moles.

Step 3: Find smallest ratio by dividing the number obtained in Step 2 by the smallest number of all.

Step 4: Write empirical formula
Laboratory Activity 3.5.1: Determining the empirical formula of copper oxide
Laboratory Activity 3.5.2: Determining the empirical formula of magnesium oxide

Worked-example 3.5(a)

In an experiment, a metal X combines with 4.32g oxygen to form 13.66g of a metallic oxide of X. Find the empirical formula of the metallic oxide.[RAM: X = 52, O = 16]
Element X O
Mass 13.66 – 4.32 = 9.34g 4.32g
Number of moles 9.34 52=0.18 4.32 16=0.27
Ratio 0.18 0.18=1.00 0.27 0.18=1.50
Simplest ratio 2 3
Empirical formula of compound is X2O3

Worked-example 3.5(b)

A compound CxHyOz contains 40% carbon and 53.3% oxygen. If the relative molecular mass of the compound is 180, find its
(a) empirical formula
(b) molecular formula
[RAM: H, 1; C, 12; O = 16]

(a) Empirical formula
Element C H O
Mass 40% 100 - 53.5 - 40 = 6.7% 53.5%
Number of moles 40 12=3.33 6.7 1.0=6.7 53.3 16.0=3.33
Ratio 3.33 3.33=1.0 0.27 0.18=1.5 3.33 3.33=1.0
Simplest ratio 1 2 1
Empirical formula of compound is C H2O

(b) Molecular formula
Assume that the molecular formula of compound CxHyOz is (CH2O) n
The relative molecular mass of X is 180.
(12 + (2×1) + 16)n = 180
30n = 180
n = 6
Thus the molecular formula of compound is (CH2O) 6 or C6H12O6
Ionic Formulae
■ Method of writing ionic formulae

Can be written if the charge of the cation(positively charged ion) and the anion(negatively charged ion) forming the ionic compound are known.

Cation Anion
Charge + Charge 2+ Charge 3+ Charge 4+ Charge 1- Charge 2- Charge 3-
IMPORTANT NOTE: MEMORIZE ALL the charges of cation and anion.
■ To write the chemical formula of an ionic compound, the following steps can be used.

(a) Write the formulae of the ions involved in forming the compound and their charges.

(b) Then balances the positive and negative charges.

(c) Finally write the chemical formula of the ionic compound without the charges.
■ The animation below shows the method of writing chemical formulae.
Naming of Chemical compounds
■ Naming of chemical compound

Based on the recommendation of the International Union of Pure and Applied Chemistry(IUPAC) .
■ Naming of ionic compound

The name of the cation comes first, followed by the name of the anion.
Cation Anion Name of ionic compound
Sodium ion Chloride ion Sodium chloride
Magnesium ion Carbonate ion Magnesium carbonate

Roman numerals are used in naming of the metals than form more than one type of ions.
Cation Anion Name of ionic compound
iron(II) ions, Fe2+ Oxide ion Iron(II) oxide
iron(III) ions, Fe3+ Oxide ion Iron(III) oxide
■ Naming of molecular compound

In naming of the simple molecular compound, the name of the first element comes first, followed by the second element with an 'ide'.:
HCl – Hydrogen chloride ; HI – Hydrogen iodide
CO – Carbon monoxide ; CO2 – Carbon dioxide.

Greek prefix are used to show the number of atoms of each element in a compound.

⇲ For exercise(objective and subjective), download for free on Android OS.

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