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The limiting molar (equivalent) ionic conductances at 25 deg C for a series of straight-chain carboxylate (fatty acid) anions are: formate HCO2- = 52, acetate CH3CO2- = 40.9, propionate CH3CH2CO2- = 35.8, butyrate CH3(CH2)2CO2- = 32.6, valerate CH3(CH2)3CO2- = ~29, and caproate CH3(CH2)4CO2- = ~28 (all in ohm⁻¹ cm² eq⁻¹). Which statement best explains this trend?

1. All these anions carry the same single negative charge, so the steady decrease in conductance with chain length is due to the increasing size of the ion, which lowers its ionic mobility.
2. The conductance decreases because the negative charge becomes progressively larger down the series.
3. The conductance decreases because the anions become more strongly hydrated as the charge density increases with chain length.
4. The conductance increases with chain length because larger ions move faster through the solvent.

Correct answer: All these anions carry the same single negative charge, so the steady decrease in conductance with chain length is due to the increasing size of the ion, which lowers its ionic mobility.

Solution

Every member of the series is a carboxylate RCO2- carrying exactly one unit of negative charge. Since the charge is constant, the variation in conductance must come from a different factor. As the alkyl chain lengthens, the ion becomes physically larger (and drags a larger solvation shell), so its mobility through the solution falls. Lower mobility means lower limiting ionic conductance, which is exactly the monotonic decrease 52 -> 40.9 -> 35.8 -> 32.6 -> 29 -> 28 observed.

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