# 109.4g Mg contains how many moles?

Is it not $24.31 \cdot g \cdot m o {l}^{-} 1$. How did I know this? Did I know the mass from the top of my head?
And so we take the quotient, $\text{Mass of stuff"/"molar mass of stuff}$
$\equiv \frac{109.4 \cdot \cancel{g}}{24.31 \cdot \cancel{g} \cdot m o {l}^{-} 1} = 4.50 \cdot \frac{1}{\frac{1}{m o l}} = 4.50 \cdot m o l$