1. Determine the moles of each element:

* Silicon (Si):

- Molar mass of Si = 28.09 g/mol

- Moles of Si = (6 g) / (28.09 g/mol) = 0.214 mol

* Chlorine (Cl):

- Molar mass of Cl = 35.45 g/mol

- Moles of Cl = (2.5 g) / (35.45 g/mol) = 0.0706 mol

2. Find the mole ratio:

* Divide the number of moles of each element by the smallest number of moles:

- Si: 0.214 mol / 0.0706 mol ≈ 3

- Cl: 0.0706 mol / 0.0706 mol ≈ 1

3. Determine the empirical formula:

* The mole ratio gives us the empirical formula: Si₃Cl

4. Calculate the empirical formula mass:

* (3 x 28.09 g/mol) + (1 x 35.45 g/mol) = 123.72 g/mol

5. Determine the molecular formula:

* Divide the given molecular mass by the empirical formula mass:

- 378 g/mol / 123.72 g/mol ≈ 3

* Multiply the subscripts in the empirical formula by this factor:

- Si₃Cl x 3 = Si₉Cl₃

Therefore, the molecular formula of the substance is Si₉Cl₃.