After completing this lesson, the student will be able to:
what are the simplest components of wood, rocks and living organisms? This is an age-old question. Ancient Greek Philosophers believed that everything was made of an elemental substance. Some believed that substance to be water, other thought it was air. Some other believed that there were four elemental substances.
As 19" century began, John Dalton proposed an atomic theory. This theory led to rapid progress in chemistry. By the end of the century however, further observations exposed the need for a different atomic theory. 20" century led to a picture of an atom with a complex internal structure.
A major goal of this chapter is to acquaint you with the fundamental concepts about matter. In this chapter you will learn some basic definitions to understand matter. This knowledge will help you in grade XI.
Remember that the chemical formula of a compound tells you which elements it contains and the whole number ratio of those atoms. In a chemical formula, the elemental symbol and numerical subscripts indicate the type and number of each atom in the compound. The compound has several chemical formulas. Learn about the two types of chemical formulas.
The empirical formula of a compound is the chemical formula that gives the simplest integer ratio of the atoms of each element. For example, the compound hydrogen peroxide has one H atom for every O atom. Therefore, the simplest ratio of hydrogen to oxygen is 1:1. The empirical formula for hydrogen peroxide is therefore written as HO.
The simplest ratio between C, H and O atoms in glucose is 1: 2: 1. What is the empirical formula of glucose?
A molecular formula is an expression that specifies the number of atoms of each element in one molecule of a compound. This shows the actual number of atoms in each molecule. For example, the molecular formula of hydrogen peroxide is H₂O,. It shows that each molecule of hydrogen peroxide is made up of two hydrogen atoms and two atoms of oxygen. Similarly C₂H₂O, is molecular formula of glucose. It shows that every molecule of glucose has 6 carbon atoms, 12 hydrogen atoms and 6 oxygen atoms. The molecular formula of a compound shows the simplest ratio between different atoms present in the compound.
Benzene is a compound of carbon and hydrogen. It contains one C atom for every H atom. Each benzene molecule actually has six carbon atoms and six hydrogen atoms. Identify the empirical and molecular formulas of benzene from the following formulas.
CH
CH
Molecular formulas for water and carbon dioxide are H₂O and CO, respectively. What are empirical formulas for these compounds?
For many compounds, empirical and molecular formulas are same. For example water (H₂O), carbon dioxide (CO,) ammonia (NH,), methane (CH,), sulphur dioxide (SO,) etc. Can you show it why?
| Element | Formula | Compound | Formula |
|---|---|---|---|
| Hydrogen | H₂ | Water | H₂O |
| Oxygen | O₂ | Carbon dioxide | CO₂ |
| Nitrogen | N₂ | Hydrochloric acid | HCL |
| Fluorine | F₂ | Sodium hydroxide | NaOH |
| Chlorine | Cl₂ | Copper (II) sulphate | CuSO₄ |
| Bromine | Br₂ | Glucose | C₆H₁₂O₆ |
Molecular mass is the sum of the atomic masses of all the atoms present in the molecule. All you have to do is to add up the atomic masses of all the atoms in the compound. For example, Molecular mass of water H₂O = 2(atomic mass of H) + atomic mass of oxygen 2(1.008)+16.00 = 2.016+16.00 = 18.016amu
Problem solving strategy:
Multiply atomic masses of carbon, hydrogen and oxygen by their subscripts and add.
Solution:
The term molecular mass is used for molecular compounds. Whereas, the term formula mass is used for ionic compounds. Ionic compounds consist of arrays of oppositely charged ions rather than separate molecules. So we represent an ionic compound by its formula unit. A formula unit indicates the simplest ratio between cations and anions in an ionic compound. For example, the common salt consists of Na⁺ and Cl⁻ ions. It has one Na⁺ ion for every Cl⁻ ion. So formula unit for common salt is NaCl.
The sum of the atomic masses of all the atoms in the formula unit of a substance is called formula mass.
Problem solving strategy:
Add the atomic masses of all the atoms in the formula unit.
Solution: