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Ch.1 question help

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Four identical containers under the same conditions are filled with gases. 

A: Nitrogen

B: Oxygen

C: Ethane

D : Neon

Which container and contents will have the highest mass?

ANS: B, Oxygen

How do you know which is the highest mass? I thought it is based on the Ar, but I was wrong. 

Please help me understand the reason why it is oxygen. 

Thank you 

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The short answer: It's because oxygen is diatomic. Let me explain what that means. The question mentions "the same conditions", which implies that the pressure (P), volume (V), and temperature (T) are the same for all of the containers. So by using the formula PV = nRT, the number of moles of the gas molecules (denoted "n" in the formula) must be the same for all containers. Hence, the answer must be the gas that has the highest molecular mass (not atomic mass).

  • Nitrogen is diatomic (N2). Thus the molecular mass is 14*2 = 28.
  • Oxygen is diatomic(O2). Thus the molecular mass is 16*2 = 32.
  • Ethane (C2H6) has the molecular mass of 12*2 + 6 = 30.
  • Neon is a noble gas (i.e. monatomic), thus will have the molecular mass of 20.

Oxygen has the highest mass (32), so it must be the answer. Pardon me for not including any units; I often find that tedious.

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