Given, P = 0.1 bar

V = 34.05 mL = 34.05 × 10^{-3}L

= 34.05 × 10^{-3} dm^{3}

R = 0.083 bar dm^{3}K^{-1}mol^{-1}

T = 546°C = (546 + 273) K = 819 K

The number of moles (n) can be calculated using the ideal gas equation as:

From the gas equation

PV = w ,RT/M,we get

M = w. RT/ PV …(1)

Substituting the given values in the equation (1), we get M = (0.0625 / 0.1 × 34.04) × 82.1 × 819 = 124.75 g/mol

Hence, the molar mass of phosphorus is 124.75 g mol^{-1}.