The number of oxygen molecules present in 16.0g of the gas is ______? [Avogadro's number = 6.02 x 1023]

Analyze the question: We need the number of O₂ molecules in 16.0 g of oxygen gas. The question says "oxygen molecules," so we use the molar mass of O₂ (not O).

Concept:

• Number of moles = mass ÷ molar mass
• Number of molecules = moles × Avogadro's number (6.02 × 10²³)

Solution:

1. Molar mass of O₂ = 2 × 16 = 32 g/mol

2. Moles of O₂ = 16.0 ÷ 32 = 0.5 mol

3. Number of molecules = 0.5 × 6.02 × 10²³ = 3.01 × 10²³ molecules

Tip: Watch the wording. "Oxygen molecules" means O₂ (32 g/mol), while "oxygen atoms" means O (16 g/mol). If the question had asked for atoms in 16.0 g, the answer would be 6.02 × 10²³ (option B), which is the trap here. Always check whether the question asks for atoms or molecules, especially with diatomic gases like O₂, N₂, H₂ and Cl₂.