The gas which will have a similar rate of effusion with CO at 350k is N2.
The rate of effusion is directly proportional to the molar mass of the gas.
The molar mass of CO is :
Mass of Carbon + Mass of oxygen
12 + 16 = 28g
Similarly finding the molar mass of N2 :
2 * Mass of Nitrogen
2 * 14 = 28g
Since the mass of both the gases are same, they'll have a similar rate of effusion.
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