# How many moles are in a 42.5 g sample of magnesium oxide MgO?

Apr 9, 2017

Approx. $1.0 \cdot m o l$

#### Explanation:

Oxygen has an atomic mass of $16.0 \cdot g \cdot m o {l}^{-} 1$; magnesium has an atomic mass of $24.3 \cdot g \cdot m o {l}^{-} 1$. We add these masses together, and we get a molar mass for $M g O$ of $40.3 \cdot g \cdot m o {l}^{-} 1$.

How many $M g O$ formula units does such a mass represent?

To find the molar quantity, we simply divide the mass by the molar quantity, to get an answer in $\text{moles}$:

$\text{Moles of MgO}$ $=$ $\frac{42.5 \cdot \cancel{g}}{40.3 \cdot \cancel{g} \cdot m o {l}^{-} 1} = 1.05 \cdot m o l$.

The answer is dimensionally consistent; we wanted an answer in moles. We got an answer in $\frac{1}{m o {l}^{-} 1} = \frac{1}{\frac{1}{m o l}} = m o l$ as required..........

How many formula units of $M g O$ does such a mass represent?