Irrespective of the source, pure water contains _________ of oxygen by mass.

88.89%

The correct answer is option 2. 88.89%.

Water has the chemical formula \(H_2O\), which means each water molecule consists of two hydrogen atoms (\(H\)) and one oxygen atom (\(O\)). To determine the percentage of oxygen in pure water by mass, we can use the molar masses of oxygen and hydrogen.

1. Molar Mass Calculation:

The molar mass of oxygen (\(O\)) is approximately \(16.00 \, \text{g/mol}\). The molar mass of hydrogen (\(H\)) is approximately \(1.01 \, \text{g/mol}\).

2. Molar Mass of Water (\(H_2O\)):

The molar mass of water is calculated by adding the molar masses of two hydrogen atoms and one oxygen atom.

\(2 \times 1.01 \, \text{g/mol (for hydrogen)} + 16.00 \, \text{g/mol (for oxygen)} = 18.02 \, \text{g/mol}\).

3. Percentage Composition Calculation:
To find the percentage of oxygen in water, we use the formula:
\[\text{Percentage of oxygen} = \frac{\text{Molar mass of oxygen}}{\text{Molar mass of water}} \times 100\]
\[\frac{16.00 \, \text{g/mol}}{18.02 \, \text{g/mol}} \times 100 \approx 88.89\%\]

Therefore, irrespective of the source, pure water contains approximately \(88.89\%\) of oxygen by mass. This percentage represents the relative mass of oxygen compared to the total mass of a water molecule.