F.6 頭chem

Mole ratio C : H = (100 - 20)/12 : 20/1 = 6.67 : 20 = 1 : 3
Hence, empirical formula = CH3

Pressure, P = (640/760) x 101000 = 85050 Pa
Volume, V = 250 cm³ = 250 x 10¯⁶ m³
No. of moles, n = ?
Gas constant, R = 8.314 J mol¯¹ K¯¹
Absolute temperature, T = 273 + 27 = 300 K

PV = nRT
(85050)(250 x 10¯⁶) = n(8.314)(300)
n = 0.008525 mol

Let (CH3)y be the molecular formula of the compound.
Molar mass: (12 + 1x3)y = 0.256/0.008525
15y = 30
y = 2

Hence, molecular formula = C2H6

by PV=mRT/M
640/760x101325Nm^-2 x 250x10^-6m^3 = [0.256g x 8.314JK^-1mol^-1 x (27+273)K]/M
M=30gmol^-1
as the compound contains 80% carbon and 20% hydrogen of mass
hence in 1 mole of the compound
mass of carbon = 24g = 2 mole of carbon atoms (24g/12gmol^-1)
mass of hydrogen = 6g = 6 mole of hydrogen atoms (6g/1gmol^-1)
hence the chemical formula is C2H6 propane