Where in the atom is most of its mass concentrated? what is this mass composed of?

The atom is mostly empty space and most of its mass is concentrated in the middle. Protons and Neutrons are responsible for most of the mass as electrons weigh close to nothing compared to these two. 

Hope I helped!

Ch4 is nonpolar and insoluble in water. since all of the hydrogen atoms are evenly spread around the carbon atom even through the c h bond is polar all the poles cancel each other out.

The formula is (nh4)2co3