Engineering Chemistry S Datta

198 ENGINEERING CHEMISTRY
Some conjugate acid-base pairs are:
HSO 4 – /SO 4 = , HNO 3
/NO 3 – , H 2
O/OH – , H 3
O + /H 2
O.
NH 4 + /NH 3
and so on.
If the acid is strong, its conjugate base will be weak and vice-versa.
Q. 5. Show that solvent acts as a base or an acid according to Lowry Bronsted
concept.
Ans. According to Lowry Bronsted concept acid–base reaction always follows the equation:
Acid I
+ Base II
Acid II
+ Base I
CH 3
COOH + H 2
O H 3
O + + CH 3
COO –
HF + CH 3
COOH CH 3
COOH 2
+
+ F –
H 2
O + H 2
O H 3
O + + OH –
So, every acid–base reaction proceeds according to the equation i.e., conjugate acids and
bases are formed during the reaction,
Acid I
+ Base II
Acid II
+ Base I
Q. 6. Give Lewis concept of acids and bases.
Ans. An acid is a species which can accept a pair of electrons and a base is a species
which can donate a pair of electrons.
..
H ←⎯ : O H – ••
..
acid
base
H
..
H: N ..
: ⎯→ H
+
H acid
base
Cl
⏐
Cl⎯B
←⎯
⏐
Cl
acid
H
⏐
: N⎯H
⏐
H
base
Q. 7. Show that ionic product of water (K w
) is 1 × 10 –14 , given K for water =
1.8 × 10 –16 .
Ans. H 2
O + H 2
O H 3
O + + OH –
or H 2
O H + + OH –
K =
+ −
[H3O ] [OH ]
[H O]
2
or
[H 2
O] = 1000
18 = 55.5555 moles l–1 .
+ −
[H ] [OH ]
[H O]
K w
= K [H 2
O] = [H + ] [OH – ] = 1.8 × 10 –16 × 55.5555
= 100 × 10 –16 = 1.00 × 10 –14 .
Q. 8. In pure water, what will be the [H + ] or [OH – ]?
Ans. As, [H + ] [OH – ] = 10 –14 and in pure water, [H + ] = [OH – ]
∴ [H + ] = [OH – ] = 10 –7 g.ion l –1 .
2