Answer :
There are several information's already given in the question. Based on those information's, the answer can be easily deduced.
Gas constant = 0.0821
Now
PV=nRT
(1.2) * (0.09) = n * (0.0821) * (297)
n = 0.00443
Mass of Butane = 32.105 - 31. 844
= 0.261
Then
Molar mass of Butane gas = 0.261/.00443
= 58.92
Gas constant = 0.0821
Now
PV=nRT
(1.2) * (0.09) = n * (0.0821) * (297)
n = 0.00443
Mass of Butane = 32.105 - 31. 844
= 0.261
Then
Molar mass of Butane gas = 0.261/.00443
= 58.92