Question

For a molecule A, the plot of total polarisation Pm vs 1/T has zero slope while for molecule B, it has a finite positive slope. Out of CHCl3 and CCl4, which one could be molecule A and why ?

Answer 1

Answer on Question #77131, Chemistry / Inorganic Chemistry

Question:

For a molecule A, the plot of total polarization $P_m$ vs 1/T has zero slope while for molecule B, it has a finite positive slope. Out of $\mathrm{CHCl}_3$ and $\mathrm{CCl}_4$, which one could be molecule A and why?

Solution:

The relation between molar polarization and temperature:

$$P_m = \frac{N_A}{3\varepsilon_0} \cdot \left(\alpha + \frac{\mu^2}{3kT}\right)$$

where $\alpha$ is the polarizability, $\mu$ is the dipole moment.

As we can see, if the dipole moment $\mu = 0$ then the second part of the sum $= 0$. Therefore $P_m$ does not depend on temperature and the plot ($P_m$ vs 1/T) has zero slope. Out of given two molecules only $\mathrm{CCl}_4$ has dipole moment $\mu = 0$.

So the molecule A is $\mathrm{CCl}_4$

Answer:

A - $\mathrm{CCl}_4$

B - $\mathrm{CHCl}_3$

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