Answer
$$n = 2.1 \space mol$$
Work Step by Step
1. According to the Ideal Gas Law:
$$n = \frac{PV}{RT} = \frac{( 1.8 \space atm )( 28.5 \space L )}{( 0.08206 \space atm \space L \space mol^{-1} \space K^{-1} )( 298 \space K)}$$
$$n = 2.1 \space mol$$
