From the ideal gas law,
n = PV / RT
n= moles = W / M
W= weight
M = molar mass
So, W / M = PV /RT
=> M = WRT / PV
We got above the formula for molar mass.
Now, put the appropriate values.
W= 0.8 g
R=0.0821 atm / K-mol
T= 373 K
V= 256mL = 0.256 L
P = 750 torr = 750 / 760 = 0.987 atm
After putting these values and calculating the value of M, we get :
M = 97 g / mol
Now, Assuming the molecular formula is [tex](CHCl)_n[/tex]
So, molar mass = n × (12.01 + 1.01 + 35.45)
equate it with 97.
We get the value of n then:
n×(12.01 + 1.01 + 35.45) = 97
n = 2
Means molecular formula is [tex]C_2H_2Cl_2[/tex]