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, C_p, of a solid can be expressed as C_p = N middot 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 middot K^-1 g^-1. Use Dulong and Petit's rule to determine the empirical formula of the compound.

Explanation / Answer

N = atoms per formula unit

R = universal gas constant

Cp = 0.21 J/Kg

apply equatin for empirical fomula...

Thallium + Chlorine

TlCl; TlCl2, TlCl3

N = 2; 3, 3, 4

MW = 239.8365 275.289, 444.219

aply:

Cp = N/MW*3*8.314

for...

0.21 = N/MW*3*8.314

N/MW = 0.21 /(3*8.314) = 0.008419

2/(239.8365 ) = 0.008419

0.00833 = 0.008419

so

TlCl is the correct value

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