When fe(no3)2(aq) and na2s(aq) are mixed, what is the black coloured precipitate that forms?

The black coloured precipitate that form is IRON ll SULPHIDE, FeS.
Iron sulphide ll sulphide is black in colour and is insoluble in water. It possess very high melting temperature and react in acids. Iron ll sulphide is used as pigment in hair dyes, glasses, bottles and paints. It is also used as lubricant.

For cyclohexane compounds drawn in a chair conformation:   -axial bonds are vertical  -equatorial bonds are parallel to two bonds within the ring.  thus, as i assume the chair conformation is drawn:   -the x group is in an axial position, as are the five other vertical bonds.  -three axial bonds are on the top face of the ring  -the other three axial bonds are on the bottom face  the position of the second substituent on each ring carbon is equatorial.  gauche groups must be on an adjacent carbon such as carbon 6 and carbon 2, and also gauche groups are rotated by 60 degrees relative to each other.  drawing the newman projections for c1–c2 and c1–c6 aids in visualizing the groups that are gauche to the y group on c1.  click this link and watch some academic videos that may you more understand the topic