Question

An element has ccp packing with a face-centered cubic unit cell. Its density is 1540 kg/m3 and the unit cell volume is 1.73 x 10-22 cm3. Calculate the molar mass (g/mol) of the element to three significant figures.

CCP: cubic close pack
You have the density (1.54 g/cc) and the volume of the unit cell (1.73 x 10^-22 cc). From mass = volume x density you can calculate mass of atoms in one unit cell. Then you know [(4 atoms/unit cell x molar mass)/6.02 x 10^23] = mass atoms. You may want to check Avogadro's number to three s.f.
Determine molar mass from this.
The 4 atoms.unit cell is where I got confused. How do you know that? And how does it change for hexagonal close pack and body centered cubic?

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