An unknown monatomic gas diffuses twice as fast as Br2. What is the identity of this unknown gas?

Answer : The unknown monoatomic gas is, argon.

Explanation :

According to the Graham's law, the rate of effusion of gas is inversely proportional to the square root of the molar mass of gas.

or,

      ..........(1)

where,

= rate of effusion of unknown gas  = 2R

= rate of effusion of gas  = R

= molar mass of unknown gas  = ?

= molar mass of gas = 160 g/mole

Now put all the given values in the above formula 1, we get:

From the molar mass 40 g/mol we conclude that the unknown monoatomic gas will be, argon.

Therefore, the unknown monoatomic gas is, argon.

it is the most commonly accepted one