Question

a gaseous compound contain 33% silicon and 67% fluorine by mass, at 35 degree. 0.210L of the compound exert a pressure of 0.2atm. if the mass of the 0.210l of the compound was 2.38g,

1) calculate the empirical formula of the compound

2) calculate the relative molecular mass of the compound

3) calculate the molecular formula of the compound

Answer 1

n_Si = 33.0g Si x $\frac{1mol Si}{28.09gSi}$ = 1.17 mol Si ÷ 1.17 = 1 mol Si

n_F = 67.0g F x $\frac{1molF}{19.00gF}$ = 3.53 mol F ÷ 1.17 = 3 mol F

empirical formula is $\implies$ SiF₃

relative molecular mass of the compound SiF₃

₌ 28.09 + 3(19) = 28.09 + 57 = 85.09 = 85.1 g

with volume and mass we can find gas density to solve for the molar mass

M = $\frac{dRT}{P}$

density of gas = 2.38 g ÷ 0.210 L = 11.33 g/L

M = $\frac{dRT}{P}$ = $\frac{(11.33g/L)(0.0821)(308K)}{0.2atm}$ = 1432.49 g /mol

empirical formula SiF₃ = mm =85.1 g

$\frac{M}{mm}$ = $\frac{1432.49}{85.1}$ ≅ 16

molecular formula is Si₁₆F₄₈

molecular mass = 1361.44 g