**Answer:**

TlCl

**Explanation:**

The Dulong and Petit's rule is:

Cp = N*3R, where Cp is the heat capacity, N is the number of atoms in the molecule, and R is the gas constant (8.314 J/K.mol). Note that **Cp has unity in gram and R in mol, so we need to transform Cp to unity in mol**.

Let's suppose that the formula of the compound formed bt thallium (Tl) and chlorine (Cl) is **TlCl**, which has molar mass: 204.4 g/mol of Tl + 35.5 g/mol of Cl = 239.9 g/mol.

**Cp observed** = 0.21 J/K.g * 239.9 g/mol = 50.379 J/K.mol

For the compound, N = 2:

**Cp theorical** = 2*3*8.314 = 49.884 J/K.mol

For **TlCl₂** the molar mass is: 204.4 + 2*35.5 = 275.4 g/mol

**Cp observed** = 0.21 * 275.4 = 57.834 J/K.mol

N = 3

**Cp theorical** = 3*3*8.314 = 74.826 J/K.mol

For **Tl₂Cl** the molar mass is: 2*204.4 + 35.5 = 444.3 g/mol

**Cp observed** = 0.21*444.3 = 93.303 J/K.mol

N = 3

**Cp theorical** = 3*3*8.314 = 74.826 J/K.mol

Note that **the difference will be greater when there are more atoms in the compound**, so the empirical formula must be TlCl which has the closest Cp observed and theoretical.