Percent oxygen by mass in sodium hydroxide (NaOH):\nCalculate the mass percent of oxygen in NaOH.

Introduction / Context:
Elemental mass percent in a compound is a foundational concept for stoichiometry, analytical chemistry, and process formulation. Here we compute the percentage of oxygen in sodium hydroxide (NaOH).

Given Data / Assumptions:

• Molar masses (approximate): Na = 23 g/mol, O = 16 g/mol, H = 1 g/mol.
• Compound: NaOH.
• Round-number atomic masses are acceptable for this calculation.

Concept / Approach:
Mass percent of an element = (mass of that element in one mole of the compound / molar mass of the compound) * 100%. For NaOH, we add atomic masses to get the formula mass, then compute the oxygen fraction.

Step-by-Step Solution:

Verification / Alternative check:
Check that the three mass percentages for Na, O, and H sum to 100%: Na = 23/40 = 57.5%; O = 40%; H = 2.5%; total = 100%.

Why Other Options Are Wrong:

• 1, 10, 16, 25: These values do not correspond to the correct mass fraction from the formula mass breakdown.

Common Pitfalls:
Using atomic percent instead of mass percent; mis-adding atomic masses; forgetting to multiply by 100.

Final Answer:
40