Which of the following contains exactly Avogadro's number of atoms?

• A. 18 g of water
• B. 1 g of hydrogen
• C. 32 g of oxygen gas
• D. 12 g of carbon-12

Answer: (D)

Verified step by step guidance

1. Recall that Avogadro's number, \$6.022 \times 10^{23}$, represents the number of particles (atoms, molecules, or ions) in one mole of a substance.
2. Understand that one mole of any element contains exactly Avogadro's number of atoms. For carbon-12, by definition, one mole corresponds to exactly 12 grams.
3. Check the molar masses of the substances given: water (H\(_2\)O) is about 18 g/mol, hydrogen (H\(_2\)) is about 2 g/mol, oxygen gas (O\(_2\)) is about 32 g/mol, and carbon-12 is exactly 12 g/mol.
4. Calculate the number of moles in each given mass by using the formula: \(\text{moles} = \frac{\text{mass}}{\text{molar mass}}\).
5. Identify which given mass corresponds to exactly one mole (i.e., contains Avogadro's number of atoms). Since 12 g of carbon-12 is defined as one mole, it contains exactly Avogadro's number of atoms.