# How many moles are in 68.5 liters of oxygen gas at STP?

It is a given that the molar volume of an ideal gas at STP is $22.414$ $L \cdot m o {l}^{-} 1$. This would probably even appear on a test paper as supplementary info.
$\frac{68.5 \cdot \cancel{L}}{22.414 \cdot \cancel{L} \cdot m o {l}^{-} 1}$ $=$ $\text{approx. } 3 \cdot m o l$.