[VISUAL] According to the information in the table above, a 1.00 g sample of which of the following — Stoichiometry Chemistry Question

STEPS:

1. Understand the relationship between chemical formula and oxygen content: Each of the given metal oxides ($\text{Na}_2\text{O}$, $\text{MgO}$, $\text{K}_2\text{O}$, and $\text{CaO}$) contains exactly one mole of oxygen atoms per mole of the formula unit.
2. State the mathematical expression for the mass of oxygen in a sample: For a $1.00\text{ g}$ sample of any of these compounds, the moles of the compound present is calculated by:
$$\text{Moles of compound} = \frac{1.00\text{ g}}{\text{Molar Mass of compound}}$$
Because there is a 1:1 molar ratio between the compound and the oxygen atoms, the moles of oxygen atoms is also equal to $\frac{1.00\text{ g}}{\text{Molar Mass of compound}}$. Thus, the mass of oxygen in a $1.00\text{ g}$ sample is:
$$\text{Mass of O} = \frac{1.00\text{ g}}{\text{Molar Mass of compound}} \times 16.0\text{ g mol}^{-1}$$
3. Identify the underlying chemistry concept (Percent Composition): This question tests the concept of percent composition by mass. The mass of oxygen in a $1.00\text{ g}$ sample is directly proportional to the mass fraction of oxygen in the compound ($\frac{16.0\text{ g/mol}}{\text{Molar Mass}}$). Therefore, the compound with the lowest overall molar mass will have the highest mass fraction of oxygen, meaning its $1.00\text{ g}$ sample will contain the greatest absolute mass of oxygen.
4. Compare the molar masses from the provided table: From the exam's reference table:
* $\text{Na}_2\text{O}$: $62.0\text{ g mol}^{-1}$
* $\text{MgO}$: $40.3\text{ g mol}^{-1}$
* $\text{K}_2\text{O}$: $94.2\text{ g mol}^{-1}$
* $\text{CaO}$: $56.1\text{ g mol}^{-1}$
5. Select the correct compound: Since $\text{MgO}$ has the lowest molar mass ($40.3\text{ g mol}^{-1}$), it has the largest proportion of oxygen by mass ($\frac{16.0}{40.3} \approx 39.7\%$). A $1.00\text{ g}$ sample of $\text{MgO}$ will therefore contain the greatest mass of oxygen ($\approx 0.40\text{ g}$), confirming Option B is correct.

*

WHY_OTHERS_WRONG:

• A is incorrect: $\text{Na}_2\text{O}$ has a higher molar mass ($62.0\text{ g mol}^{-1}$) than $\text{MgO}$. This means a $1.00\text{ g}$ sample contains fewer moles of the compound and thus less oxygen by mass ($\approx 0.26\text{ g}$).
• C is incorrect: $\text{K}_2\text{O}$ has the highest molar mass among the options ($94.2\text{ g mol}^{-1}$) due to the heavy potassium atoms. Consequently, it has the lowest percent composition of oxygen by mass, with a $1.00\text{ g}$ sample containing only $\approx 0.17\text{ g}$ of oxygen.
• D is incorrect: $\text{CaO}$ has a molar mass of $56.1\text{ g mol}^{-1}$, which is larger than that of $\text{MgO}$. This results in a lower oxygen mass percent than $\text{MgO}$, yielding only $\approx 0.29\text{ g}$ of oxygen in a $1.00\text{ g}$ sample.