Answer
$$168.5 \space g/mol$$
Work Step by Step
1. Convert the pressure to Pa:
$$p = 20 \space kPa= 20 \times 10^3 \space Pa = 20000 \space Pa$$
2. Calculate the molar volume.
$$pV_m = RT$$ $$V_m = \frac{RT}{p} = \frac{(8.31447 \space Pa \space m^3 \space K^{-1} \space mol^{-1})(330 \space K)}{20000 \space Pa}$$ $$V_m = 0.137 \space m^3 \space mol^{-1}$$
3. Find the molar mass:
$$0.137 \space m^3 \space mol^{-1} \times 1.23 \space kg \space m^{-3} = 0.1685 \space kg \space mol^{-1}$$ $$0.1685 \times 10^3 \space g \space mol^{-1} = 168.5 \space g/mol$$
