What is the molecular formula of SxFy ?

Question

General formula : SxFy

One of the compound is 25.23%S. If you place 0.0955 of the compound in a 89 mL flask at 45 degree celcius, the pressure of the gas is 83.8mmHg. What is the molecular formula ?

Answer 1

A standard analysis can show that the empirical formula must be   SF₅ .     Therefore, the most likely candidate is

 

 S₂F₁₀   .  The molar mass of  S₂F₁₀  is 254 g.   Thus  .0955 g is  .000376 moles.    If one assumes that S₂F₁₀  would be in the gas phase at 45C, the ideal gas law (PV = nRT) can be used with   n = .000376,  V = .089 L,  T = 318K  to compute the pressure.   The result is about .11 atm which is  83.6 mmHg.  This is pretty close to 83.8.  

 

Thus one concludes that the answer is  S₂F10.  

 

The standard analysis for the empirical formula is :   Consider a 100 g sample.   Since the %S is  25.23, the hypothetical sample would contain 25.23 g S.   Since the molar mass of S is 32,  this is .788 mole S.    The remaining 74.8 g is F.   Since the molar mass of F is 19,   this is 3.93 moles F.   The ratio  3.93 / .788 ~ 5  ,  so the empirical formula is  SF₅.    An molecule with an odd number of F  would have an unpaired electron - not likely.