Which of the following has the highest molar conductivity?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → potassium hexacyanoferrate(II)
The molar conductivity of a solution of a complex depends on the number of ions each formula unit gives in water, and on their charges. The counter ions outside the square bracket separate as free ions; the complex ion stays as one unit. Werner used such conductivity measurements to find the ionisable part of complexes.
Option (1) Potassium hexacyanoferrate(II), \( \ce{K4[Fe(CN)6]} \): \( \ce{K4[Fe(CN)6] -> 4K+ + [Fe(CN)6]^4-} \): 4 + 1 = 5 ions.
Option (2) Hexaaquachromium(II) chloride, \( \ce{[Cr(H2O)6]Cl2} \): \( \ce{[Cr(H2O)6]Cl2 -> [Cr(H2O)6]^2+ + 2Cl-} \): 1 + 2 = 3 ions.
Option (3) Tetraamminedichloridocobalt(III) chloride, \( \ce{[Co(NH3)4Cl2]Cl} \): \( \ce{[Co(NH3)4Cl2]Cl -> [Co(NH3)4Cl2]+ + Cl-} \): 1 + 1 = 2 ions.
Option (4) Potassium tetracyanidonickelate(II), \( \ce{K2[Ni(CN)4]} \): \( \ce{K2[Ni(CN)4] -> 2K+ + [Ni(CN)4]^2-} \): 2 + 1 = 3 ions.
Potassium hexacyanoferrate(II) gives the largest number of ions, and the most highly charged complex ion, so it has the highest molar conductivity.