A $2.1\ \mathrm g$ sample of a liquid vaporizes and exerts $120\ \mathrm{mmHg}$ pressure at $1.5\ \mathrm L$ volume and $80\ \mathrm{^\circ C}$ temperature. Find the molar mass of the gas.
I first get the equation $PV=nRT$ and replace $n$ with $m/M$ since $M=m/n$ Then the equation becomes this $$PV=\frac{mRT}{M} \Rightarrow M=\frac{mRT}{PV}$$
$$\begin{align} M&=\frac{mRT}{PV}\\[6pt] \Rightarrow M&=\frac{(2.1\ \mathrm g)\times(62.364\ \mathrm{L\ mmHg/(mol\ K}))\times(80+273.13)\ \mathrm K}{(120\ \mathrm{mmHg})(1.5\ \mathrm L)}\\[6pt] \Rightarrow M&=257\ \mathrm{g/mol} \end{align}$$
But the answer key says $150\ \mathrm{g/mol}$. What am I doing wrong?