Which gas is more dense at standard temperature and pressure (STP), sulfur hexafluoride (SF_6) or helium (He), and what evidence supports your answer?

• A. Both gases have the same density at STP because all gases occupy the same volume.
• B. Sulfur hexafluoride (SF_6) is less dense because it contains more atoms per molecule.
• C. Helium (He) is more dense because it is a noble gas and does not react with other substances.
• D. Sulfur hexafluoride (SF_6) is more dense because it has a much higher molar mass than helium (He).

Answer: (D)

Verified step by step guidance

1. Recall that at standard temperature and pressure (STP), all ideal gases occupy the same volume (22.4 L per mole), so volume does not affect density differences between gases.
2. Understand that the density (\(d\)) of a gas can be calculated using the formula \(d = \frac{m}{V}\), where \(m\) is mass and \(V\) is volume. Since volume is constant for one mole at STP, density depends directly on molar mass.
3. Calculate or compare the molar masses of sulfur hexafluoride (SF\(_6\)) and helium (He). For SF\(_6\), sum the atomic masses: 1 sulfur atom plus 6 fluorine atoms. For helium, use the atomic mass of a single helium atom.
4. Recognize that sulfur hexafluoride has a much higher molar mass than helium, which means one mole of SF\(_6\) weighs significantly more than one mole of He, even though both occupy the same volume at STP.
5. Conclude that because density is mass per unit volume and SF\(_6\) has a higher molar mass, sulfur hexafluoride is more dense than helium at STP.