1.

Which of the following statements best explains why a closed balloon filled with He gas rises in air?

Answer»

He is monoatomic gas, where as nearly all molecules that make up air are diatomic eg:
`N_(2),O_(2)` etc.
`U_(avg)` of He is higher than `U_(avg)` of air molecules and higher speeds of COLLISIONS with the balloon walls proped the balloon UPWARD
Because the He atoms are lighter than average air molecules, as a result He (g) is less DENSE than air. The mass of balloon is thus less than the mass of air displaced by its volume.
Because he has lower molar mass that the average air molecules, the He atoms are in faster motion. This means temperature of He is higher than the air temperature, hot gases tend to rise.

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