Which of the following steps correctly converts 18.0 grams of water (H_2O) to the number of molecules?

• A. Multiply 18.0 g directly by Avogadro's number.
• B. Multiply 18.0 g by the molar mass of H_2O, then divide by Avogadro's number.
• C. Divide 18.0 g by Avogadro's number, then multiply by the molar mass of H_2O.
• D. Divide 18.0 g by the molar mass of H_2O, then multiply by Avogadro's number.

Answer: (D)

Verified step by step guidance

1. Identify the given quantity and what you need to find: You have 18.0 grams of water (H\_2O) and want to find the number of molecules.
2. Recall the molar mass of water (H\_2O), which is the sum of the atomic masses of 2 hydrogen atoms and 1 oxygen atom. This value is approximately 18.0 g/mol.
3. Convert grams of water to moles by dividing the mass by the molar mass: \(\text{moles} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}}\).
4. Use Avogadro's number (\$6.022 \times 10^{23}\( molecules/mol) to convert moles to molecules by multiplying the number of moles by Avogadro's number: \)\text{molecules} = \text{moles} \times 6.022 \times 10^{23}$.
5. Combine the steps into one expression: \(\text{molecules} = \left( \frac{18.0\ \text{g}}{18.0\ \text{g/mol}} \right) \times 6.022 \times 10^{23}\).