We know 1 mole of any atom or molecules contains 
atom or molecules.
1 mole of HBr i.e 81 gm/mol contains atom or molecules.
So, mass of 
molecules is :
Therefore, mass of molecules is 1.21 gm .
Hence, this is the required solution.
D. 8
You can predict it from it's name "Octet" means Eight. Atom must have that number of electron to have a stable structure!
Hope this helps!
molar mass of methane CH4
= C + 4 H
= 12.0 + 4 x 1.008
= 12.0 + 4.032
= 16.042g/mol
7.31 x 10^25 molecules x <u> 1 mole CH4 </u> = 121.43 moles
6.02 x 10^23 CH4 molecules
121.43 moles CH4 are present.
C. The action/reaction law states that a force is equal in both directions.
