The French chemists Pierre L. Dulong and Alexis T. Petit noted in 1819 that the

Question

The French chemists Pierre L. Dulong and Alexis T. Petit noted in 1819 that the molar heat capacity of many solids at ordinary temperatures is proportional to the number of atoms per formula unit of the solid. They quantified their observations in what is known as Dulong and Petit's rule, which says that the molar heat capacity, CP, of a solid can be expressed as Cp= N*3R

Where N is the number of atoms per formula unit and R is the universal gas constant.

The observed heat capacity per gram of a compound containing thallium and chlorine is 0.21 J·K–1·g–1. Use Dulong and Petit's rule to determine the empirical formula of the compound.

Explanation / Answer

According to Dulong–Petit rule

Cp (J/k.g) = 3 R (J/k.mole) * N

Cp (J/k.g) * M (g/mole) = 3 R (J/k.mole) * N - Converting Cp from J/k.g to J/k.mole

Where M is the molecular weight of thallium chloride

So

M/N = 3 * 8.314 / 0.21

M/N = 118.77

So if we take N =2

Then M = 237.54

This is close to molar mass of TlCl which is 239.82

So the empirical formula of thallium chloride is TlCl

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