What is the molar mass of the halide

Question

A sample of a metal halide forms a rock-salt type of unit cell of density 18.1 g/cm3. The

sum of the radii of the cation and the anion is 215 pm. What is the molar mass of the

halide?

A. 123 g/mol

B. 265 g/mol

C. 217 g/mol

D. 26.0 g/mol

E. 79.8 g/mol

im not sure how to do this

i saw an equation

l^3d=(# of atoms/U.C.)(MM)/Na

but i couldnt understand the equation

Answer 1

http://www.uwgb.edu/DutchS/PETROLGY/dens...

first let's get the length of a side of the unit cell = diameter of anion plus diameter of cation = 430 picometres

430 pico metres = 4.3e-8 cm

now let's get the volume of the unit cell:

(4.3e-8 cm)^3 = 7.95e-23 cm3

next let's get the mass of the unit cell using the density:

7.95e-23 cm3 @ 18.1 g / cm3 = 1.439 e-21 grams

now let's change grams into amu's using Avagadros #:

1.439 e-21 grams @ 6.022e23 amu/ gram = 866 amu's per unit cell

now let's get the mass of the molecule, knowing that the unit cell of  "a rock-salt type of unit cell" has 4 molecules each of metal halide:

866 amu's / cell @ 4 molecules per cell = 216.5 amu's per molecule

if there are 216.5 amu/ molecule, .... then there is 216.5 greams / mole

your answer is:C. 217 g/mol