Answer
$$27.9 \mathrm{\ g} / \mathrm{mol}
$$
Work Step by Step
We find $M$ from $\rho=p M / R T$ with $T=273 \mathrm{K}$.
$$
M=\frac{\rho R T}{p}=\frac{\left(0.0124 \mathrm{kg} / \mathrm{m}^{3}\right) 8.31 \mathrm{J} / \mathrm{mol} \cdot \mathrm{K}(273 \mathrm{K})}{1.01 \times 10^{3} \mathrm{Pa}}$$$$=0.0279 \mathrm{kg} / \mathrm{mol}$$$$=27.9 \mathrm{g} / \mathrm{mol}
$$
