element which can be used to stuff
bottles that enclose ancient paper
must be a gas at room temperature
must be denser than helium
must not react with other elements
28 is correct. (NH4)2 is equal to 10 atoms. (8 H and 2 N ) CO3 is equal to 4 atoms. (1 C and 3 O) That's 14 atoms. But it's two molecules of this. So you just multiply by the number of molecules (2), and you get 28.