- Each element is represented by its own chemical symbol.
- In an element, the chemical formula represents the atoms in the molecule.
- The chemical formula tells us:
- the types of atoms or ions in the compound,
- the number of atoms or ions in the compound,
- For example, the chemical formula for ethene is C2H4. This shows that ethene is the result of the combination of the elements carbon and hydrogen, and there are 2 carbon atoms and 4 hydrogen atoms in each molecule of ethene.
- Generally, chemical formula can be divided into
- Empirical formula
- Molecular formula
- The empirical formula of a substance is the chemical formula that gives the simplest whole-number ratio of atoms of each element in the substance.
- The molecular formula of a substance is the chemical formula that gives the actual number of atoms of each element in the substance.
Empirical Formula
- The empirical formula of a substance is the chemical formula that gives the simplest whole-number ratio of atoms of each element in the substance.
- Empirical = information gained by means of observation, experience, or experiment.
Chemical Substances | Molecular Formula | Empirical Formula |
Glucose |
C6H12O6 |
CH2O |
Water |
H2O |
H2O |
Carbon Dioxide |
CO2 |
CO2 |
Benzene |
C6H6 |
CH |
Butane |
C4H8 |
CH2 |
Finding Empirical Formula
Steps to determine the empirical formula of a compound STEP 1: Find the mass STEP 2: Find the mole STEP 3: Find the simplest ratioElement |
Fe |
O |
Mass |
4.23g |
1.80g |
Number of mole |
4.23 56 =0.0755 |
1.80 16 =0.1125 |
Simple ratio |
0.0755 0.0755 =1 |
0.1125 0.0755 =1.5 |
Ratio in round number |
2 |
3 |
Element |
Mg |
S |
O |
Percentage |
20.2% |
26.6% |
53.2% |
Mass in 100g |
20.2g |
26.6g |
53.2g |
Number of mole |
20.2 24 =0.8417mol |
26.6 32 =0.8313mol |
53.2 16 =3.325mol |
Simple ratio |
0.8417 0.8313 =1 |
0.8313 0.8313 =1 |
3.325 0.8313 =4 |
Element |
C |
H |
Percentage |
85.7% |
14.3% |
Mass in 100g |
85.7g |
14.3g |
Number of mole |
85.7 12 =7.142mol |
14.3 1 =14.3mol |
Simple ratio |
7.142 7.142 =1 |
14.3 7.142 =2 |
Molecular Formula
- The molecular formula of a substance is the chemical formula that gives the actual number of atoms of each element in the substance.
- A molecular formula is the same as or a multiple of the empirical formula.
- For example, the empirical of carbon dioxide is CO2 and the molecular formula is also CO2.
- Whereas, the empirical formula of ethane is CH3 while the molecular formula of ethane is C2H6.
Finding Molecular Formula
Given that the empirical formula of benzene is CH and its relative molecular mass is 78. Find the molecular formula of benzene. [Relative Atomic Mass: Carbon: 12; Hydrogen: 1]
Answer:
Let’s say the molecular formula of benzene is CnHn.
The relative molecular mass of CnHn
= n(12) + n(1)
= 13n
13n = 78
n = 78/13 = 6
Therefore, the molecular formula of benzene
= C6H6
Example:
What is the mass of metal X that can combine with 14.4g of oxygen to form X oxide with molecular formula X2O3. (RAM: O = 16; X = 56 )
Answer:
Number of mole of oxygen = 14.4 16 =0.9 mol From the molecular formula, we learn that the ratio of element X to oxygen X:O = 2:3 Therefore, the number of mole of X =0.9× 2 3 =0.6 mol Number of mole, n = mass Molar mass 0.6= mass 56* mass=33.6g The mass of element X = 33.6g *Molar mass of a substance = Relative atomic mass of the substance
Percentage of Composition of a Compound
- To find the percentage of composition of a substance means to find the percentage of mass of each element in the molecule of the substance to the mass of the molecule.
- The percentage of mass of an element can be determined by the following equation:
- Percentage of mass of an element = Sum of the Relative Atomic Mass of the Element Relative Molecular Mass of the substance ×100%
Example
Calculate the percentage of composition of DDT (C14H9Cl5). [Relative atomic mass: Carbon = 14; Hydrogen = 1; Chlorine = 35.5]
Answer:
Relative molecular mass of DDT =14(12)+9(1)+5(35.5) =354.5
Percentage of carbon = 14(12) 354.5 ×100%=47.4%
Percentage of hydrogen = 9(1) 354.5 ×100%=2.5%
Percentage of chlorine = 5(35.5) 354.5 ×100%=50.0%