# How many moles of CO_2 gas are contained in a flask holding 55.0 g of gas?

$\text{Number of moles}$ $=$ $\text{Mass of gas"/"Molar mass of gas}$
$\text{Number of moles}$ $=$ $\frac{55.0 \cdot \cancel{g}}{44.01 \cdot \cancel{g} \cdot m o {l}^{-} 1} = 1.25 \cdot m o l$
Note that our mathematical expression was consistent dimensionally, the $g$ cancelled out, and $\frac{1}{m o {l}^{-} 1} = \frac{1}{\frac{1}{m o l}} = m o l$ as required.