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The root mean square speeds at STP for the gases H2, N2, O2 and HBr are in the order:

1. H2 < N2 < O2 < HBr
2. HBr < O2 < N2 < H2
3. H2 < N2 = O2 < HBr
4. HBr < O2 < H2 < N2

Correct answer: HBr < O2 < N2 < H2

Solution

The root mean square speed of a gas is inversely proportional to the square root of its molar mass. Since H2 has the smallest molar mass and HBr the largest, the order is HBr < O2 < N2 < H2.

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