Question

a. (5 points) Calculate the molar mass (i.e., average molecular weight) of the mixture of 80 mol% methane, 10 mol% ethane, and 10 mol% of propane. b. (5 points) Calculate the specific gravity of this mixture. You may assume that air and the gas are ideal at the standard conditions.

Answer 1

a) Let there be total 100 moles total:

Moles of methane = 80, ethane = 10, propane = 10

Avg molar weight = $\sum$ (Number of moles * Molar mass) / Total number of moles

= (80*16 + 10*30 +10*44)/100 = 20.2 g/mol

b)Using PM=dRT (ideal gas equation) P is pressure, d is density, M is avg molar weight, R is gas constant and T is temp

( d  = PM/RT = (1atm * 20.2g/mol) / (0.0821 atm L K^-1 mol^-1 * 273 K) = 0.901 g/L = 0.000901g/cm³

So, Specific gravity wrt air= d/ density of air = 0.000901g/cm³ / 0.001225 g/cm³= 0.7355 (Answer)