$$ \begin{array}{|c|c|} \hline
\text{Compound} & \text{Molar Mass} \\
& \text{(grams)} \\ \hline \hline
\ce{Na2O} & 62.0 \\ \hline
\ce{MgO} & 40.3 \\ \hline
\ce{K2O} & 94.2 \\ \hline
\ce{CaO} & 56.1 \\ \hline
\end{array} $$
According to the information in the table above, a 1.00 g sample of which of the following contains the
greatest mass of oxygen?
We can find which compound has the greatest mass of oxygen by percent.
$$ \text{Percent Oxygen} = \frac{\text{mass of oxygen}}{\text{molar mass}} $$
\(\ce{Na2O}\)
$$ \text{Percent Oxygen} = \frac{16}{62} $$
\(\ce{MgO}\)
$$ \text{Percent Oxygen} = \frac{16}{40.3} $$
\(\ce{K2O}\)
$$ \text{Percent Oxygen} = \frac{16}{94.2} $$
\(\ce{CaO}\)
$$ \text{Percent Oxygen} = \frac{16}{56.1} $$
Since the numerator is the same for all fractions, the largest value will result from the smaller denominator value.