Air molecules are constantly striking your face. How fast are they moving when they collide with you? Calculate the root mean square speed of nitrogen molecules at 20. °C.

ANTICIPATE Because a sound wave is propagated by the movement of molecules, you should expect that typical speeds are about the same as the speed of sound in air. That is about 300 m · s^–1.

PLAN For the actual calculation, substitute the data into Eq. 7a. In calculations of basic physical properties such as speed, use R in SI units and molar masses in SI base units, namely, kilograms per mole.

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Urms = 3RT M (7a)