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Question

The values of Λ0eq for NH4Cl, NaOH and NaCl are 149.74, 248.1 and 126.4 ohm1 cm2 eq1 respectively. The value of Λ0eq(NH4OH) is :

A
371.44
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B
271.44
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C
171.44
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D
Cannot be calculated from the data given
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Solution

The correct option is B 271.44
Kohlrausch law:
At infinite dilution, when all the interionic effects disappear and dissociation of electrolyte is complete, each ion makes a definite contribution towards equivalent conductance of the electrolyte irrespective of the nature of the ion with which it is associated.
Kohlrausch law of independent migration of ions:
Λ0eq=λ0eq++λ0eq

λ0eq+Limiting equivalent conductivities of cation
λ0eqLimiting equivalent conductivities of anion

Λ0NH4Cl=λ0eq(NH+4)+λ0eq(Cl)

Λ0eqNaOH=λ0eq(Na+)+λ0eq(OH)

Λ0eqNaCl=λ0eq(Na+)+λ0eq(Cl)
Thus,
Λ0eq(NH4OH)=λ0eq(NH+4)+λ0eq(Cl)+λ0eq(Na+)+λ0eq(OH)[λ0eq(Na+)+λ0eq(Cl)]

Λ0eq(NH4OH)=λ0eq(NH4Cl)+λ0eq(NaOH)λ0eq(NaCl)

Λ0eq(NH4OH)=149.74+248.1126.4
Λ0eq(NH4OH)=271.44 ohm1 cm2 eq1

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