Grade 11 → Basic concepts of chemistry ↓
Empirical and molecular formula
The world of chemistry provides us with a systematic way to understand the composition and structure of substances. Two important concepts in this understanding are the empirical formula and the molecular formula. These formulas help us know the elements present in the compound and their proportions. Let us take a deeper look at each of these concepts to understand their importance in chemistry.
Understanding the empirical formula
The empirical formula of a compound gives the simplest whole number ratio of atoms of each element present in the compound. It does not show the actual number of atoms, but the simplest ratio of the elements.
For example, the molecular formula of the compound ethylene is C 2 H 4
The ratio of carbon atoms to hydrogen atoms is 1:2, so its empirical formula is CH 2
.
To represent the empirical formula visually, imagine a simple square representing each atom, where the carbon atoms are black and the hydrogen atoms are white:
CHH ,
Each black square is a carbon atom, and each white square is a hydrogen atom, representing their basic structure in its simplest form.
Steps to determine the empirical formula
Follow these steps to find the empirical formula of a compound:
- Determine the mass (in grams) of each element present in the compound.
- Convert these masses into moles using their atomic masses from the periodic table.
- Divide the mole value of each element by the smallest mole value obtained from your calculations.
- If necessary, multiply these numbers by integers to get whole numbers. This gives sub-numbers for each element in the empirical formula.
Example calculation
Suppose we have a compound that contains 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass. Let's find its empirical formula:
- Let's say we have 100 grams of the compound. So, we have 40.0 grams of carbon, 6.7 grams of hydrogen, and 53.3 grams of oxygen.
- Convert these masses to moles:
- Carbon:
40.0 div 12.01 approx 3.33
moles - Hydrogen:
6.7 div 1.008 approx 6.65
moles - Oxygen:
53.3 div 16.00 approx 3.33
mol - Divide each mole value by the smallest number of moles, which is 3.33:
- Carbon:
3.33 div 3.33 = 1
- Hydrogen:
6.65 div 3.33 approx 2
- Oxygen:
3.33 div 3.33 = 1
- Thus, the empirical formula is
CH 2 O
Understanding the molecular formula
The molecular formula shows the actual number of each type of atom in the molecule. While the empirical formula gives the simplest ratio, the molecular formula displays the actual composition of the molecule, providing more detailed information about its structure.
A molecular formula is an integer multiple of its empirical formula. For example, the molecular formula of hydrogen peroxide is H 2 O 2
, and its empirical formula is HO
. This means that the molecular formula is an integer multiple of the empirical formula.
The molecular formula can also be seen with squares representing the atoms. In H 2 O 2
, we have:
huh
Steps to determine molecular formula
To find the molecular formula, you'll need two pieces of information: the compound's empirical formula and molar mass. Here are the steps:
- Calculate the mass by the empirical formula.
- Divide the molar mass of the compound by the mass of the empirical formula to get a whole number that is a multiple of the empirical formula.
- Multiply the sub-digits in the empirical formula by this factor to get the molecular formula.
Example calculation
Given that the empirical formula of a compound is CH
and its molar mass is 78 g/mol, find the molecular formula:
- Calculate the mass of the empirical formula
CH
:12.01 + 1.01 = 13.02
g/mol. - Divide the molar mass of the compound by the empirical formula mass:
78 div 13.02 approx 6
. - Multiply the sub-numbers in the empirical formula by 6:
- The molecular formula is
C 6 H 6
.
Visual examples of empirical vs. molecular formulas
Consider the compound glucose with the molecular formula C 6 H 12 O 6
The empirical formula of glucose is CH 2 O
because it represents the simplest ratio of 1:2:1 for carbon, hydrogen, and oxygen.
Here's how the relationship between empirical and molecular formulas works visually:
Molecular: CCCCCC , hhhhhhh , Ooooo Empirical: CHHO (repeated block) ,
Importance in chemistry
Both formulas are essential in the study and application of chemistry:
- Empirical formula: This is used to determine the simplest structure, which is important in understanding how compounds react and interact at the most basic levels.
- Molecular formula: Provides a complete picture for scientists wishing to synthesize chemicals or understand the function of a compound in biological systems.
Practice problems
1. A compound has a percentage composition of 85.7% carbon and 14.3% hydrogen. Determine its empirical formula.
2. A sample of an unknown compound has a molar mass of 180 g/mol and its empirical formula CH 2 O
What is the molecular formula of the compound?
Conclusion
Understanding empirical and molecular formulas is a crucial step in the study of chemistry. By mastering these concepts, both students and professionals can understand the nature of compounds, leading to further discovery and innovation. Through both simple calculations and in-depth analysis, these formulas lay the groundwork for exploring the astonishing complexity of chemical interactions and their effects on the world.