Which statement is true at standard temperature and pressure (STP)? (The atomic mass of Mg is 24.31 u.)

• A. One mole of Mg occupies 22.4 L at STP.
• B. One mole of Mg contains 6.022 × 10^{23} atoms and has a mass of 24.31 g.
• C. The molar volume of solid Mg at STP is 22.4 L.
• D. At STP, 24.31 g of Mg contains 1.00 × 10^{23} atoms.

Answer: (B)

Verified step by step guidance

1. Recall that at standard temperature and pressure (STP), the molar volume of an ideal gas is 22.4 L per mole. This applies specifically to gases, not solids or metals like magnesium (Mg).
2. Understand that one mole of any element contains Avogadro's number of atoms, which is \(6.022 \times 10^{23}\) atoms. This is a fundamental concept in chemistry.
3. Use the atomic mass of magnesium (Mg), which is given as 24.31 u, to determine the mass of one mole of Mg atoms. By definition, one mole of Mg atoms has a mass of 24.31 grams.
4. Recognize that the molar volume of solid magnesium is not 22.4 L because this volume applies only to gases at STP. Solids have much smaller molar volumes due to their dense packing of atoms.
5. Calculate the number of atoms in a given mass of Mg by using the ratio: number of moles = mass / molar mass, then multiply by Avogadro's number to find the number of atoms. For example, 24.31 g of Mg corresponds to one mole, which contains \(6.022 \times 10^{23}\) atoms, not \(1.00 \times 10^{23}\) atoms.