Question

At $STP$ the order of root mean square velocity of molecule of $H_2$, $N_2$, $O_2$ and $HBr$ is :

• A. $H_2$ > $N_2$ > $O_2$ > $HBr$
• B. $HBr$ > $O_2$ > $N_2$ > $H_2$
• C. $HBr$ > $H_2$ > $O_2$ > $N_2$
• D. $N_2$ >$O_2$ > $H_2$ > $HBr$

Answer 1

The correct option is A $H_2$ > $N_2$ > $O_2$ > $HBr$

As we know,

$U_{rms}$ = $\sqrt{\frac{3RT}{M_w}}$

Molecular weight order

$H_2$ < $N_2$ < $O_2$ < $HBr$
$2$ $28$ $32$ $81$

So, $rms$ speed order is

$H_2$ > $N_2$ > $O_2$ > $HBr$

Hence, the correct option is $A$