Question

Name two coordination compounds which are important in biological systems.

Hint:

Chlorophyll (Mg), haemoglobin (Fe), vitamin B₁₂ (Co).

Answer 1

Concept: Kohlrausch's law lets us build up the conductivity of a whole electrolyte (when it is infinitely diluted) by simply adding the separate contributions of its positive and negative ions. The shape of the conductivity graph is also different for strong and weak electrolytes, and that difference is the key to this question.

Step 1: State the law in words
Kohlrausch's law of independent migration of ions says that at infinite dilution (when the solution is so dilute that the ions move totally freely and do not disturb each other), the limiting molar conductivity of an electrolyte is the sum of the limiting molar conductivities of its cation and its anion, each counted as many times as it appears: \[ \Lambda^\circ_m = \nu_+ \lambda^\circ_+ + \nu_- \lambda^\circ_- \] Here $\nu_+$ and $\nu_-$ are the numbers of cations and anions per formula unit.

Step 2: The curve for a strong electrolyte
If you plot molar conductivity $\Lambda_m$ against $\sqrt{c}$ for a strong electrolyte (say KCl), you get an almost straight line. Strong electrolytes are fully broken into ions even at higher concentrations, so the line is steady and gentle. You can simply extend (extrapolate) this straight line back to where $c = 0$ and read off $\Lambda^\circ_m$ from the graph. Easy and reliable.

Step 3: The curve for a weak electrolyte
For a weak electrolyte, like acetic acid, the story is different. At normal concentrations only a small fraction of it splits into ions, so its molar conductivity stays low. But as you keep diluting, more and more of it breaks into ions, and very close to infinite dilution the molar conductivity shoots up very steeply. This makes the curve bend sharply upward near the $\Lambda_m$ axis instead of staying straight. Because the curve rises so steeply and is not a straight line near $c = 0$, you cannot extend it to find $\Lambda^\circ_m$ by extrapolation. So instead we calculate $\Lambda^\circ_m$ for the weak electrolyte indirectly, by adding the known ionic values using Kohlrausch's law.

Answer: $\Lambda^\circ_m = \nu_+ \lambda^\circ_+ + \nu_- \lambda^\circ_-$. For weak electrolytes the conductivity curve rises too steeply near zero concentration to be a straight line, so it cannot be extrapolated; we use Kohlrausch's law with ionic values instead.