Page 22 - Chemistry ICSE Class IX

10 ICSE Chemistry – 9

EXAMPLE 1.1 Give the names of the elements present in the following
compounds:
(a) Quicklime (b) Hydrogen bromide
(c) Baking powder (d) Potassium sulphate
Solution:
Compound Formula Elements present in the compound

(a) Quicklime CaO Calcium, Oxygen
(b) Hydrogen bromide HBr Hydrogen, Bromine
(c) Baking powder NaHCO 3 Sodium, Hydrogen, Carbon, Oxygen

(d) Potassium sulphate K SO 4 Potassium, Sulphur, Oxygen
2
Writing the Molecular Formulae of Molecular
Compounds

How is the molecular formula of a molecular compound
written
The molecular formula of a molecular compound is written as follows:
Step 1. Write the symbols of the constituent elements side by side, in such a
dŚĞ ĐŽŵƉŽƵŶĚƐ ĨŽƌŵĞĚ ďLJ ĐŽǀĂůĞŶƚ way that the less electronegative element is on the left and the more
ďŽŶĚŝŶŐ ĂƌĞ ĐĂůůĞĚ ŵŽůĞĐƵůĂƌ electronegative element is on the right.
ĐŽŵƉŽƵŶĚƐ͘
ͻ Step 2. Write their valency numbers over the symbols and criss-cross the
dŚĞ ŶƵŵďĞƌ ŽĨ ĞůĞĐƚƌŽŶƐ ĂŶ ĂƚŽŵ valency numbers to write as subscripts to the symbols.
ƐŚĂƌĞƐ ƚŽ ĨŽƌŵ ĐŽǀĂůĞŶƚ ďŽŶĚƐ ǁŝƚŚ Step 3. Divide these numbers (written as subscripts) by a common factor, if
ŽƚŚĞƌ ĂƚŽŵƐ ŝƐ ĐĂůůĞĚ ŝƚƐ ĐŽŵďŝŶŝŶŐ needed.
ĐĂƉĂĐŝƚLJ Žƌ ǀĂůĞŶĐLJ͘
This method of writing the molecular formula of a molecular
compound is illustrated through the following examples:
ƒ Water – Water contains hydrogen and oxygen. Oxygen is more
electronegative than hydrogen. So,
Step 1. Write the symbols of hydrogen and oxygen side-by-side with
symbol of oxygen on the right.
H O
Step 2. Write the valencies of hydrogen and oxygen in water.
H 1+ O 2–
Step 3. Criss-cross the valency numbers.
1+
H o o O 2–
dŚĞ ĂƐƐŝŐŶŵĞŶƚ ŽĨ ŽdžŝĚĂƟŽŶ
ŶƵŵďĞƌ ƚŽ ĂƚŽŵƐ ŝŶ ĐŽǀĂůĞŶƚ H O o H O
2 1
2
ŵŽůĞĐƵůĞƐ ĂŶĚ ƉŽůLJĂƚŽŵŝĐ ĂŶŝŽŶƐ ŝƐ So, the molecular formula of water is H O.
Ă ĨŽƌŵĂůŝƚLJ ƚŽ ŚĞůƉ ǁƌŝƟŶŐ ĨŽƌŵƵůĂĞ 2
ĂŶĚ ďĂůĂŶĐŝŶŐ ĐŚĞŵŝĐĂů ĞƋƵĂƟŽŶƐ͘ ƒ Hydrogen chloride – Hydrogen chloride contains hydrogen and
&Žƌ ƚŚŝƐ ƉƵƌƉŽƐĞ͕ ƚŚĞ ĞůĞĐƚƌŽŶƐ chlorine. Chlorine is more electronegative than hydrogen. Then by
ƐŚĂƌĞĚ ďĞƚǁĞĞŶ ƚǁŽ ƵŶůŝŬĞ ĂƚŽŵƐ following the steps given above, one can write
ĂƌĞ ĐŽƵŶƚĞĚ ĂƐ ŝĨ ƚƌĂŶƐĨĞƌƌĞĚ ƚŽ ƚŚĞ Step 1. H Cl
ŵŽƌĞ ĞůĞĐƚƌŽŶĞŐĂƟǀĞ ĂƚŽŵ͘
1+
Step 2. H o o Cl 1–
Step 3. H Cl 1 or HCl
1
So, the molecular formula of hydrogen chloride is HCl.