Chemical Foemulae and Equation

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Card 1: State the meaning of chemical formula.

Chemical Formula

A chemical formula is a symbol use to show the type of elements and quantity of element that present in a chemical substance.

Formula of water is H2O.

The formula shows that

  • There are 2 hydrogen atoms and 1 oxygen atom present in 1 water molecule.
Card 2: State the meaning of empirical formula.

Empirical Formula

An empirical formula is a chemical formula that gives the simplest whole-number ratio of each element in a molecule.

A glucose molecule consist of 6 carbon atoms, 12 hydrogen atom and 6 oxygen atoms.

The ratio of carbon:hydrogen:oxygen = 6:12:6
The simplest ration = 1:2:1

Therefore, the empirical formula of glucose is CH2O

Example 1
Calculate the empirical formula of a compound consisting of 85.7% C and 14.3% H. (Relative atomic mass: Carbon=12; Hydrogen = 1)


Element Carbon (C) Hydrogen(H)
Percentage 85.7% 14.3%
Mass in 100g 85.7g 14.3g
Number of mole 85.7/12=7.142 mol 14.3/1 = 14.3
Simple ratio 7.142/7.142 = 1 14.3/7.142 = 2

The simplest ration of Carbon and Hydrogen is 1:2. Therfore the empirical formula = CH2


Card 3: State the meaning of molecular formula.

Molecular Formula

A molecular formula is a chemical formula that shows the actual number of each element in a molecule.

Card 4: Constructing Chemical Formula

Constructing Chemical Formula

2 Requirements to form the formula of an ionic compound

  • Have at least 2 types of ions that contain opposite charge.
  • The amount of positive charge/charges must be equal to the amount of negative charge/charges in the compound.


1 Magnesium ion combine with 2 chloride ions to form 1 magnesium chloride formula.

Writing Magnesium Chloride Formula

Card 5: State the meaning of chemical Equation.

Chemical Equation

A chemical equation is a precise description of a chemical reaction.


2 sodium atoms react with 2 water molecules to form 2 sodium hydroxide molecule and 1 hydrogen molecule.

Equantion: 2Na + 2H2O ---> 2NaOH + H2

Reactants: Na and H2O
Products: NaOH and H2

SPM Physics in 3 Days