Concise Physical Chemistry

c13 JWBS043-Rogers September 13, 2010 11:27 Printer Name: Yet to Come

PROBLEMS AND EXAMPLES 217

(b) Given the additional information that ◦AgNO 3 = 133. 3 × 10 −^4 ohm−^1

m^2 mol−^1 , what are the molar ionic conductivities and mobilities of Ag+(aq)

and NO− 3 (aq)?

Solution 13.3

(a) If the Ag+(aq) ions did not move, the amount of silver ion found in the

cathode compartment after electrolysis would have been 0.0900 mols because

of the removal of 0.0100 mol of Ag+(aq) by electrodeposition. The fact that

there are 0.0947 mol in the cathode compartment after deposition means that

0.0047 mol of Ag+(aq) have been replaced by Ag+(aq) migration from the

center compartment. The transport number is the ratio of the charge transferred

by Ag+(aq) transport relative to the total charge transferred by both ions:

t+=

0. 0047

0. 0100

= 0. 47

The rest of transport must have been due to NO− 3 (aq) ions migrating in the

opposite direction, sot−= 1. 00 − 0. 47 = 0 .53.

(b) Transport numbers are related to ionic conductances according to the equations

t+=

λ◦+

◦

t−=

λ◦−

◦

which is reasonable because an ion that transports more than its share of

charge will have a large ionic conductivity relative to the total◦. Given that

◦AgNO 3 = 133. 3 × 10 −^4 ohm−^1 m^2 mol−^1 , we obtain

λ◦+=t+◦= 0. 47

(

133. 3 × 10 −^4

)

= 62. 6 × 10 −^4 ohm−^1 m^2 mol−^1

and

λ◦−=t−◦= 0. 53

(

133. 3 × 10 −^4

)

= 70. 6 × 10 −^4 ohm−^1 m^2 mol−^1

Problem 13.1

A solution containing 1. 155 × 10 −^2 mol of HCl was neutralized by adding NaOH

solution. The process brought about a 0.553 K temperature rise. An electrical heater

of 58.7-ohm resistance brought about a 0.487 K temperature rise in 15.0 min. A

potential of 6.03 volts was maintained across the resistor by a potentiostat. What is

the molar enthalpy of HCl? Remember that a joule is a volt coulomb.