The density of chromium metal is 7 g cm$$^{-3}$$. If edge length of unit cell is 300 pm, identify the type of unit cell. (At mass Cr = 52)

How many gram of H$$_2$$O are present in 0.25 mol of it?

Identify molecular formula of pyridine from following

Which of the following is a first step in mechanism of heterogenous catalysis?