Engineering Chemistry S Datta

IONIC EQUILIBRIUM 199
Q. 9. What is pH?
Ans. It is negative logarithm of hydrogen ion concentration, i.e.,
pH = – log 10
[H + ] or [H + ] = 10 –pH
The acidity or alkalinity of a solution is generally expressed by pH.
Q. 10. Explain the pH scale.
Ans. The pH scale is a logarithmic scale for measuring the concentration of hydrogen
ions in aqueous solutions.
In acidic solutions, [H + ] range = 10 0 – 10 –6 g . ion l –1
In neutral solutions, [H + ] = [OH – ] = 10 –7 g . ion l –1 .
In alkaline solutions, [H + ] range = 10 –8 – 10 –14 g . ion l –1 .
pH = – log 10
[H + ]
pH = 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
[H + ] = 10 0 10 –1 10 –2 10 –3 10 –4 10 –5 10 –6 10 –7 10 –8 10 –9 10 –10 10 –11 10 –12 10 –13 10 –14
acidic ←⎯⎯ neutral ⎯⎯→ alkaline
Q. 11. What is the pH of 0.02 mol dm –3 HCl?
Ans. pH = – log 10
[H + ]
= – log 10
[2 × 10 –2 ] (as the acid is strong)
= – log 10
2 + 2 log 10
10
= 2 – 0.3010 = 1.699 ≈ 1.7.
Q. 12. What is the hydrogen concentration of coca cola with pH 2.37?
Ans. pH = 2.3
or – log 10
[H + ] = 2.3
or log 10
[H + ] = – 2.3
or [H + ] = 10 –2.3
= 5 × 10 –3 mol dm –3 .
Q. 13. Explain the pH changes during acid–base titration.
Ans. Addition of an alkali from a burette to 25 ml of an acid in a conical flask and
plotting a graph give a shape determined by the nature of the acid and the base.
Phenolphthalein
pH 7
pH 7
Methyl
orange
0
0
25
Alkali(c.c.)
Strong acid–strong base
0
0
25
Alkali(c.c.)
Weak acid–strong base
Fig. 8.1 Neutralisation Curves.