A sample contains 12.0 g of water. How many moles is that, and how many water molecules are present?

• A. Moles ≈ 0.0333 mol; molecules ≈ 2.0×10^22.
• B. Moles ≈ 0.6667 mol; molecules ≈ 4.02×10^23.
• C. Moles ≈ 1.0 mol; molecules ≈ 6.0×10^23.
• D. Moles ≈ 6.022 mol; molecules ≈ 3.6×10^24.

Answer: (B)

Converting grams to the amount of substance and then to molecules uses two standard factors: the molar mass and Avogadro’s number. Water has a molar mass of about 18.0 g per mole. So the number of moles in 12.0 g is 12.0 g ÷ 18.0 g/mol ≈ 0.6667 mol. To get the number of molecules, multiply by Avogadro’s number: 0.6667 mol × 6.022×10^23 molecules/mol ≈ 4.02×10^23 molecules. This gives roughly 0.67 mol and 4.02×10^23 water molecules, which is why that answer is correct.