Page 128 - Chemistry ICSE Class X

5 Mole Concept and

Stoichiometry

The Mole Concept
CONTENTS
What is a mole
ͻ dŚĞ ŵŽůĞ ĐŽŶĐĞƉƚ
ͻ 'ĂLJͲ>ƵƐƐĂĐ͛Ɛ ůĂǁ ŽĨ ĐŽŵďŝŶŝŶŐ We buy things either by number or by mass. For example, we generally
ǀŽůƵŵĞƐ buy oranges, eggs, bananas, etc. by number, viz., by dozen (1 dozen = 12
ͻ ^ŽŵĞ ĞdžĂŵƉůĞƐ ďĂƐĞĚ ŽŶ 'ĂLJͲ pieces). We buy most of our vegetables by mass, viz., gram or kilogram.
>ƵƐƐĂĐ͛Ɛ ůĂǁ ŽĨ ĐŽŵďŝŶŝŶŐ ǀŽůƵŵĞƐ The unit we choose to express the quantity is just a matter of convenience.
ͻ ǀŽŐĂĚƌŽ͛Ɛ ůĂǁ The mole is a concept of quantity in terms of number and mass. This
ͻ ƚŽŵŝĐŝƚLJ concept relates the mass of any material to the number of atoms, ions or
ͻ ƉƉůŝĐĂƟŽŶƐ ŽĨ ƚŚĞ ǀŽŐĂĚƌŽ͛Ɛ ůĂǁ molecules in it.
ͻ ^ŽŵĞ ĞdžĂŵƉůĞƐ ďĂƐĞĚ ŽŶ
ǀŽŐĂĚƌŽ͛Ɛ ůĂǁ 6JG OQNG CU RGT FGſPKVKQP CEEGRVGF KPVGTPCVKQPCNN[ KU FGſPGF CU
ͻ ƚŽŵŝĐ ŵĂƐƐ ĂŶĚ ƌĞůĂƟǀĞ ĂƚŽŵŝĐ follows:
ŵĂƐƐ “The mole is the amount of a substance which contains the same
ͻ &ƌĂĐƟŽŶĂů ĂƚŽŵŝĐ ŵĂƐƐĞƐ number of chemical units (atoms, molecules or ions) as there are atoms
ͻ DŽůĞĐƵůĂƌ ŵĂƐƐ ĂŶĚ ƌĞůĂƟǀĞ in exactly 12 gram of pure carbon-12.”
ŵŽůĞĐƵůĂƌ ŵĂƐƐ
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ͻ &ŽƌŵƵůĂ ŵĂƐƐ 12 g of carbon-12 is found to contain 6.02 × 10 atoms of carbon-12.
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ͻ 'ƌĂŵͲĂƚŽŵŝĐ ŵĂƐƐ ĂŶĚ ŐƌĂŵͲ Thus, a mole represents a collection of 6.02 × 10 chemical units (atoms,
ŵŽůĞĐƵůĂƌ ŵĂƐƐ molecules or ions).
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ͻ ŽŵƉŽƐŝƟŽŶ ŽĨ Ă ĐŽŵƉŽƵŶĚ The number 6.02 × 10 is called Avogadro’s number. Avogadro’s
ͻ ĞƚĞƌŵŝŶĂƟŽŶ ŽĨ ĞŵƉŝƌŝĐĂů ĂŶĚ number is commonly denoted by N or N .
ŵŽůĞĐƵůĂƌ ĨŽƌŵƵůĂ A 0
ͻ ĂůĐƵůĂƟŽŶƐ ďĂƐĞĚ ŽŶ ĐŚĞŵŝĐĂů Thus, a mole is the quantity of any material which contains one
23
ĞƋƵĂƟŽŶƐ Avogadro’s number (6.02 × 10 ) of chemical units (atoms, molecules, or
ions).
Mole concept can be applied to both the molecular and ionic
substances. For example,
/Ŷ ƐƵƉĞƌŵĂƌŬĞƚƐ ŽĨ h^ ͬ ĂŶĂĚĂ ĂŶĚ
ƐŽŵĞ ƉůĂĐĞƐ ŝŶ /ŶĚŝĂ͕ ďĂŶĂŶĂƐ ĂƌĞ ƐŽůĚ One mole of CO = 1 mol of CO 2
2
23
ďLJ ǁĞŝŐŚƚ͕ ĂŶĚ ŶŽƚ ďLJ ŶƵŵďĞƌƐ ĂƐ ŝƐ = 6.02 × 10 molecules of CO
ĚŽŶĞ ŝŶ ŵŽƐƚ ƉůĂĐĞƐ͘ ^Ž͕ ƚŚĞ ĐŚŽŝĐĞ 23 2
ŽĨ ƚŚĞ ƵŶŝƚ ĚĞƉĞŶĚƐ ƵƉŽŶ ƚŚĞ ƉĞŽƉůĞ͘ = Mass of 6.02 × 10 molecules of CO 2
/Ŷ ŵƵĐŚ ƚŚĞ ƐĂŵĞ ǁĂLJ ĂƐ ǁĞ ďƵLJ Ă = 44 g of CO 2
ĚŽnjĞŶ ĞŐŐƐ Žƌ Ă ŬŝůŽŐƌĂŵ ŽĨ ƐǁĞĞƚƐ͕ Ă One mole of NaCl = 1 mol of Na Cl –
+
ĐŚĞŵŝƐƚ ĚĞĂůƐ ǁŝƚŚ Ă ŵŽůĞ ŽĨ ĂƚŽŵƐ͕
23
–
23
+
ŵŽůĞĐƵůĞƐ͕ ŝŽŶƐ Žƌ ĞůĞĐƚƌŽŶƐ͘ = 6.02 × 10 Na ions + 6.02 × 10 Cl ions
–
+
23
= Mass of 6.02 × 10 Na Cl ion pairs
= (23 + 35.5) g of NaCl
= 58.5 g of NaCl
Symbol of the mole unit
The unit of each physical quantity has been given a standard symbol. For
example, gram is denoted by g, a kilogram by kg, a metre by m, and newton
by N (Newton is the unit of force). The unit of mole is given a symbol mol.
So, if you want to express one mole, you may write it as 1 mol.