Let's assume that the gas has ideal gas behavior.
Then we can use ideal gas formula,
PV = nRT
Where, P is the pressure of the gas (Pa), V is the volume of the gas (m³), n is the number of moles of gas (mol), R is the universal gas constant ( 8.314 J mol⁻¹ K⁻¹) and T is temperature in Kelvin.
P = 60 cm Hg = 79993.4 Pa
V = 125 mL = 125 x 10⁻⁶ m³
n = ?
R = 8.314 J mol⁻¹ K⁻¹
T = 25 °C = 298 K
By substitution,
79993.4 Pa x 125 x 10⁻⁶ m³ = n x 8.314 J mol⁻¹ K⁻¹ x 298 K
n = 4.0359 x 10⁻³ mol
Hence, moles of the gas = 4.0359 x 10⁻³ mol
Moles = mass / molar mass
Mass of the gas = 0.529 g
Molar mass of the gas = mass / number of moles
= 0.529 g / 4.0359 x 10⁻³ mol
= 131.07 g mol⁻¹
Hence, the molar mass of the given gas is 131.07 g mol⁻¹