How many formula units are in the unit cell shown for li2s?

Q:108 a)&b) please explain; specifically (b).
Please.

The crystal structure of lithium sulfide (Li_2S), is pictured here. The length of the unit cell is 5.88 x 10^2 pm. For this structure, determine the coordination numbers of Li^+ and S^2- the number of formula units in the unit cell the density of Li_2S Refer to Figure 12-44 and Figure 12-48. Suppose that the two planes of icons pictured in Figure 12-44 correspond to the top and middle planes of icons in the NaCl unit cell in Figure 12-48. If the X-rays used have a wavelength of 154.1 pm, at what angle theta would the diffracted beam have its greatest intensity?

The crystal structure of lithium sulfide (Li_2S), is pictured here. The length of the unit cell is 5.88 x 10^2 pm. For this structure, determine the coordination numbers of Li^+ and S^2- the number of formula units in the unit cell the density of Li_2S Refer to Figure 12-44 and Figure 12-48. Suppose that the two planes of icons pictured in Figure 12-44 correspond to the top and middle planes of icons in the NaCl unit cell in Figure 12-48. If the X-rays used have a wavelength of 154.1 pm, at what angle theta would the diffracted beam have its greatest intensity?

Answer

108 Looking at the figure we can deduce,

(a) the coordination number of Li = 4 and S^2- = 8

(b) the number of formula units in the unit cell 4

(c) Volume of unit cell = (5.88 x 10^-8 cm)^3 = 2.03 x 10^-22
cm^3

mass of one Li2S = 45.95/6.023 x 10^23 = 7.63 x 10^-23 g

mass of 4 Li2S in unit cell = 3.05 x 10^-22 g

density = 3.05 x 10^-22/2.03 x 10^-22 = 1.50 g/cm^3