What is the approximate mole fraction of krypton (Kr) in dry air at sea level?

• A. 0.78
• B. 0.21
• C. 2.1 × 10^{-3}
• D. 1.1 × 10^{-6}

Answer: (D)

Verified step by step guidance

1. Understand that mole fraction is the ratio of moles of a component to the total moles in the mixture. For gases in air, mole fraction can be approximated by volume fraction because gases behave ideally.
2. Recall that the major components of dry air at sea level are nitrogen (N\_2), oxygen (O\_2), argon (Ar), carbon dioxide (CO\_2), and trace gases including krypton (Kr). The mole fraction of major gases like N\_2 and O\_2 are approximately 0.78 and 0.21 respectively.
3. Look up or recall the typical mole fraction of krypton in dry air, which is a trace gas present in very small amounts, on the order of parts per million or less.
4. Express the mole fraction of krypton as a decimal fraction, typically around 1.1 × 10^{-6}, which means krypton makes up about one part per million of the air by mole.
5. Compare this mole fraction to the given options to confirm that 1.1 × 10^{-6} is the correct approximate mole fraction of krypton in dry air at sea level.