Writing Chemical Formulae

The shortest way to represent a compound with the help of symbols and valency of elements is known as chemical formula.  Chemical formula of a compounds shows of each combining element.

In ionic compounds, the charge on each ion is used to determine the chemical formula of a compound.

There are some rules for writing the chemical formula

(i):- The valencies or charges on the ion must be balanced.

(ii):- When a compound consists of a metal and a non-metal, the symbol of the metal is written first and on the left whereas of non-metal on its right.

(iii):- When compound is formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio. e.g. Ca(OH)2.

Formulae of Simple Compounds

To write the chemical formula for simple compounds.

 (i):- Write the symbols of constituent elements and their valencies as shown below.

(ii):- Write the symbol of cation first followed by the symbol of anion.

(iii):- Then criss-cross their charges or valencies to get the formula.

Note:- The simplest compounds made up of two different elements are also called binary compounds.

In other words, the positive and negative charges must balance each other and the overall structure must be neutral.

Note:- When the valency of both elements are numerically equal, the subscripts are also not written.

We use brackets when we have two or more of the same ions in the formulae. For Example:-

All subscripts must be reduced to lowest term (except for molecule or covalent compound). For Example:-  

Illustration

Write symbols of the following elements:

Silver, Chromium, Chlorine, Mercury, Lead, Copper, Gold, Aluminium.

Solution

Silver (Ag), Chromium (Cr), Chlorine (CI), Mercury (Hg), Lead (Pb), Copper (Cu), Gold (Au), Aluminium (Al).

Illustration

Define atomic mass unit.

Solution

One atomic mass unit is the mass unit (u) equal to exactly 1/12th of the mass of one atom of C-12 isotope.

Illustration

How many kinds of atoms are present in a molecule of calcium carbonate (CaCO3)?

Solution

CaCO3 is a heteroatomic molecule which contains three types of atoms, i.e. one atom of calcium, one atom of carbon and three atoms of oxygen.

Molecular Mass

The molecular mass of a substance is the sum of the atomic Masses of all the atoms in a molecule of the substance. Therefore, the relative molecular mass of a molecule is its relative mass expresses in atomic mass units (u).

For example,

 The relative molecular mass of water (H2O) is 18 u, which can be calculated as

 Atomic mass of hydrogen =1 u

Atomic mass of oxygen = 16 u

H2O contains two hydrogen atoms and one oxygen atom. Therefore, molecular mass of water is

= 2x1 + 1 x 16 = 18 u

Illustration

Calculate the molar mass of the following substances:

(i):- Ammonia, (ii):- Hydrochloric acid, (iii):- Phosphorus molecule,

(iv):-Hydrogen molecule, (v):- Oxygen molecule, (vi):- Sulphur dioxide

 Solution

(i):- Molar mass of ammonia (NH3)

  =1 x 14 + 3 x 1 = 17 u

(ii):- Molar mass of hydrochloric acid (HCI)

=1 x 1 + 1 x 35.5 = 36.5 u

(iii):- Molar mass of phosphorus molecule (P4)

= 4 x 31 = 124 u

(iv):- Molar mass of hydrogen molecule (H2)

 = 2 x 1= 2 u

(v):- Molar mass of oxygen molecule (O2)

= 2 x 16 = 32 u

(vi):- Molar mass of sulphur dioxide (SO2)

 = 32 + 2 x 16 = 64 u

Formula Unit Mass

It is the sum of the atomic masses of all atoms present in a formula unit of a compound.

Formula unit mass is calculated in the same manner as we calculate the molecular mass.

e.g. formula unit mass for sodium chloride (NaCI)

= 1 x 23 + 1 x 35.5 = 58.5 u

Mole Concept

One mole of any species (atoms, molecules, ions or particles) is that quantity in number having a mass equal to its atomic or molecular mass in grams.

e.g. 1 mole of carbon (atomic mass =12) is equal to 12 g.

1 mole of oxygen (O2, molecular mass = 2 x 16) is equal to 32 g.

1 mole of water (H2O, molecular mass = 2 x 1 + 1 x 16) is equal to 18 g.

Avogadro Constant

The number of particles present in 1 mole of any substance is same and fixed, which is equal to 6.022 x 1023. This is a constant, known as Avogadro constant or Avogadro number (NA).

Thus, mole is also defined as number of particles equal to the Avogadro constant, NA(6.022 x 1023).

1 mole = 6.022 x 1023 particles, in number.

Molar Mass and Moles

The mass of 1 mole of a substance is equal to its relative atomic or molecular mass in gram. Since the atomic mass or molecular mass of an element gives us the mass of one atom of that element in atomic mass units (u). Thus, to get the mass of 1 mole of an atom of that element we have to take the same numerical value but change the units from `if to 'g‘.

          Illustration

Find the mass of :

(i):- 1 mole of nitrogen atoms, (ii):- 8 moles of aluminium atoms, (iii):- 0.2 mole of oxygen atoms, (iv):-2 moles of water molecules.

 Solution

(i):- Mass of 1 mole of nitrogen atoms

  • = molar mass in gram =14 g

(ii):- We know that,

  • mass of 1 mole of Al-atoms = 27 g
  • Mass of 8 moles of Al-atoms = 8 x 27 = 216 g

(iii):- Mass of 0.2 mole of oxygen atoms = 0.2 x 16 = 3.2 g

  • Mass of 1 mole of oxygen atoms =16 g

(iv):- Mass of 2 moles of water molecules = 2 x 18 = 36 g

  • Mass of 1 mole of water molecule =18 g

Percentage Composition

The percentage composition of an element in a compound is the percentage of the mass contributed by the element to the total mass of the compound. It is obtained by dividing mass of that element in the compound by the total mass of the compound and multiplying by 100, i.e.,.

Illustration

Calculate the percentage composition of carbon in CO2.

 Solution

Molar mass of CO2 =12 + 2 x16 = 44g mol-1

Mass due to carbon (C) = 12 g

Percentage composition of C = 12/44 x100 = 27.3%

Advanced Level

Illustration

Write the molecular formulae of all the compounds that can be formed by the 3M combination of the following ions:  

Solution

(a):- Compounds of Cu2+  =  CuCl2, CuSO4, Cu3(PO4)2

(b):- Compounds of Na+ = NaCl, Na2SO4, Na3PO4

(c):- Compounds of Fe3+ = FeCl3, Fe2(SO4)3, FePO4

Illustration

Calculate the number of moles of magnesium present in a magnesium ribbon weighing 12 g. Molar atomic mass of magnesium is 24 g mol-1.

From the table, find

(a) A pair of ions              (b) An atom of noble gas

(c) A pair of isobars         (d) A pair of isotopes

Solution

Illustration

Calculate the formula unit mass of CaCl2.

Solution

Atomic mass of Ca = 40 and Cl = 35.5

CaCl2 = 40 + 2 x 35.5 = 40 + 71 = 111u

Illustration

Give the names of any two elements present in the following compounds:

Baking powder, Common salt, Sulphuric acid

Solution

Baking powder (NaHCO3) Sodium, hydrogen, carbon and oxygen.

Common salt (NaCl) Sodium and chlorine.

Sulphuric acid (H2SO4) Sulphur, hydrogen and oxygen.

Illustration

6 g of coke consisting 100% carbon, is burnt in air. Find the number of oxygen atoms consisted by the carbon dioxide gas thus formed.

Solution

Each carbon atom combine with two oxygen atoms up on combustion as follows:    

Illustration

Find the number of atoms in 120 g of calcium and 120 g of iron. Which one has more number of atoms and how much is the difference?

[Atomic mass of Ca = 40 u, Fe = 56 u]

Solution

          Molar mass of Ca = 40 g

                    40 g of Ca contains, number of atoms = 6.022 x 1023

          Then, let the size of the nucleus be x m.

Illustration

List down three different names given to the path in which electrons revolve around the nucleus. Also explain why are they called so?

Solution

The three different names are:-

Discrete orbit It is called so because electrons revolve in certain distinct path and not just in orbit.        

Energy level (energy shell) The energy associated with different orbits (with which electron revolves) is distinct for each orbit, hence it is called so.

Stationary state Since, the energy associated with an orbit is fixed. Hence, the electron revolving in a particular orbit have stationary energy. That's why, orbit is also called stationary state.

Illustration

A metal (mass number = 40) having same number of protons and neutrons, combines with two chlorine atoms. Identify the element with which electronic configuration of this metal matches in combined state.

Solution

Mass number = Number of protons + Number of neutrons

40 = 2 x Number of protons

  Number of protons = 20

Hence, the metal is calcium (Ca).

Since, it combines with 2 Cl atoms (total valency = 2)

Hence, it will acquire 2 positive charge, after losing 2 electrons as

K L M

Electronic configuration of Ca2+ ion = 2, 8, 8

This electronic configuration matches to that of argon (Ar).