The volume of 1 mole of gas is always 22.4 dm3. Molar volume= Volume/number of moles (first we should discover the number of mole) so 22.4 =1/number of moles <=> number of moles=1/22.4 moles
Now that we know the number of moles we can use the formula Molar Mass= mass /number of moles <=> 254= mass/(1/22.4) so mass =254/22.4 <=> mass= 11.339 which is 11.3 g