Question

2. (3 points) A chloride of silicon contains 79.1 mass % Cl. A) What is the empirical formula of the chloride? B) If the molar mass is 269 g/mol, what is the molecular formula?

Answer 1

Silicon chloride is made up of Si & Cl. Therefore, we can write % of Si + % of Cl = 100 %

Hence, % of Si = 100 % - % of Cl = 100 % - 79.1 % = 20.9 %

Now, calculate moles of Si and Cl.

Moles of Si = % of Si / atomic mass of Si = 20.9 / 28.08 = 0.744

Moles of Cl = % of Cl / atomic mass of Cl = 79.1 / 35.45 = 2.23

Hence, ratio of moles of Si : Cl is

0.744 / 0.744 = 1 mol Si

2.23 / 0.744 = 2..997 $\approx$ 3 mol Cl

From above values, we get empirical formula of compound as SiCl₃.

Empirical formula mass of compound = 28.08 + ( 3*35.45) = 134.43 g / mol

We have relation, Molecular formula = r x empirical formula

Where r is the ratio of molar mass of substance to the empirical formula mass.

Therefore, r = 269 g / mol / 134.43 g/mol =2.00

Hence, Molecular formula of compound = 2.00 ( SiCl₃. ) = Si₂Cl ₆

ANSWER : Empirical formula : SiCl₃. Molecular formula : Si₂Cl ₆