The molar conductance of NaCl varies with the concentration as shown in the following table :
When a certain conductivity cell(c) was filled with 25 × 10–4(M) NaCl solution, the resistance of the cell was found to be 1000 ohm. At infinite dilution ion conductances of Cl– and SO42– are 80 ohm–1 cm2 mol–1 and 160 ohm–1 cm2 mol–1 respectively.
(i) What is the molar conductance of NaCl at infinite dilution ?
Text Solution
Verified by ExpertsCHECK THE SOLUTION.
(i)
Sol. π m = 
– b 
; c = 4 × 10–4(M) π m = 107
or 107 = 
– b × 2 × 10–2 ..... (1)
97 = 
– b × 3 × 10–2 ..... (2)
or 10 = b × 10–2 or b = 1000 and
= π m + b 
or 
= 107 + 103 × 2 × 10–2
or 
= 127 ohm–1 cm2 mol–1
(ii)
Sol. For 25 × 10–4(M) NaCl, π m = 
– b 
or π m = 127 – 103 × (25 × 10–4)1/2
or π m = 127 – 103 × 5 × 10–2 = 127 – 50
or π m = 77 ohm–1 cm2 mol–1
R = ρ × 
or 
= R × 
= 
where KC is the cell constant
or KC = κ × R
π m = 
or 77 = 
or κ = 77 × 25 × 10–7 ohm–1 cm–1
∴ KC = 77 × 25 × 10–7 × 103
= 77 × 25 × 10–4 = 0.1925 cm–1
(iii)
Sol. For Na2SO4 solution, κ = 
= 
= 4.81 × 10–4 ohm–1 cm–1
∴ π m(Na2SO4) = 
= 
or π m(Na2SO4)= 
× 4.81 × 100 ohm1 cm2 mol–1
or π m(Na2SO4) = 192.4 ohm–1 cm2 mol–1
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