How do I find the number of molecules?

Nickel crystallizes in the face-centered cubic (fcc) lattice. the density of the metal is 8902 kg/m3. calculate the radius of a nickel atom.

Asmall amount of a solid is added to water. the observation made after fifteen minutes is shown in the figure. which of these solids has been probably added to water? a) oil b) sand c) sugar d) wood chips

Which part of a feedback mechanism is able to monitor the conditions outside of cells and usually uses nerve cells to relay this information to an intergrating center