The molar mass of gas = 206.36 g/mol
Further explanation
In general, the gas equation can be written

where
P = pressure, atm
V = volume, liter
n = number of moles
R = gas constant = 0.082 l.atm / mol K
T = temperature, Kelvin
mass (m)= 2.89 g
volume(V) = 346 ml = 0.346 L
T = 28.3 C + 273 = 301.3 K
P = 760 mmHg=1 atm
The molar mass (M) :
