# How many grams are in 4.35 moles of Ar?

Approx. $170$ $g$.
Argon has a mass of $39.95$ $g \cdot m o {l}^{-} 1$. If there are 4.35 mol, then the mass is simply $4.35$ $m o l \times 39.95 \cdot g \cdot m o {l}^{-} 1$. Note that as an inert gas, we quite properly assume that argon is atomic. What assumption would we make if the gas had been oxygen or nitrogen?