The root mean square (rms) speed of a gas is given by
[tex]v_{rms} = \sqrt{ \frac{3RT}{M_m} } [/tex]
where R is the gas constant, T the absolute temperature of the gas and [tex]M_m[/tex] is the molar mass of the gas.
For molecular oxygen [tex]O_2[/tex], the molar mass is [tex]M_m = 0.032 kg/mol[/tex], so since the temperature of the gas in the problem is T=423 K, the rms speed of its molecules is
[tex]v_{rms}= \sqrt{ \frac{3 (8.31 J mol^{-1} K^{-1})(423 K)}{0.032 kg/mol} }= 574.1 m/s[/tex]
