Which species has the highest mass percent of oxygen?

• A. H2O
• B. CaCO3
• C. CO2
• D. HCO3-

Answer: (A)

To determine which species has the highest mass percent of oxygen, we need to calculate the mass percent of oxygen in each compound.

Starting with water (H2O), it contains two hydrogen atoms and one oxygen atom. The molar mass of hydrogen is approximately 1 g/mol, and for oxygen, it is about 16 g/mol. Therefore, the molar mass of H2O is:

• ( 2 \times 1 + 16 = 18 ) g/mol.

The mass contribution from oxygen is 16 g, so the mass percent of oxygen in H2O is calculated as:

• ( \frac{16 , \text{g}}{18 , \text{g}} \times 100% \approx 88.9% ).

Next, examining calcium carbonate (CaCO3), the molar mass consists of one calcium atom (approximately 40 g/mol), one carbon atom (approximately 12 g/mol), and three oxygen atoms (approximately 16 g/mol each). The calculation gives:

• ( 40 + 12 + (3 \times 16) = 100 ) g/mol.

The contribution from oxygen is ( 3 \times 16 =