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Which of the following correctly describes the conductometric titration graph when a strong acid is titrated against a weak base?

1. Conductance decreases to a minimum at the equivalence point and then increases sharply
2. Conductance remains nearly constant up to the equivalence point and then increases sharply
3. Conductance decreases to a minimum at the equivalence point and then increases slightly
4. Conductance increases rapidly at first and then becomes nearly constant after the equivalence point

Correct answer: Conductance decreases to a minimum at the equivalence point and then increases slightly

Solution

During titration of HCl (strong acid) with NH3 (weak base): HCl + NH3 -> NH4Cl. Initially, the solution has H+ (high molar conductivity ~350 S cm²/mol) and Cl⁻. As NH3 is added, H+ is neutralised and replaced by NH4+ (lower conductivity). Conductance decreases progressively. At equivalence point, only NH4Cl remains — minimum conductance. After equivalence, excess NH3 (weak base, poorly ionised) adds very few ions, so conductance increases only slightly. The graph shows: decreasing line to minimum at equivalence, then gentle upward slope. This matches option C.

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