What is the molecular formula?

Question

Chemical analysis of a gaseous compound showed that it

contained 33.0 percent silicon (Si) and 67.0 percent fluorine (F)

by mass.

At 35° C, 0.210 L of the compound exerted a pressure of 1.70

atm.

If the mass of 0.210 L of the compound was 2.38 g, calculate

the molecular formula of the compound.

Answer 1

From the data of temperature, volume and pressure, we can calculate the number of moles of the unknown, using the ideal gas equation of PV = nRT

P = 1.70 atm

V = 0.210 L

n = ?

R = 0.0821 Latm/Kmol

T = 35ºC + 273 = 308K

n = PV/RT = (1.70)(0.210)/(0.0821)(308) = 0.0141 moles of unknown

Now we can find the molar mass of the unknown: 2.38 g/0.0141 moles = 168.8 molar mass

From the % data, we can determine the empirical formula:

assuming you have 100 g of unknown...

33% Si = 0.33 x 100g = 33 g Si x 1 mole/28.09 g = 1.175 moles Si

67% F = 0.67 x 100g = 67 g F x 1 mole/19 g = 3.526 moles F

Divide both by 1.175 to get

Si = 1.00

F = 3.00

Empirical formula = SiF₃

Molar mass SiF₃ = 85

How many of these empirical formulae go into the molar mass of the unknown of 168 g/mole?

169/85 = 1.99 = 2

So, MOLECULAR FORMULA of UNKNOWN is Si₂F₆