Question

1) How many H^- ions are around each Na⁺ ion in NaH, which has a cubic unit cell with H^- ions on each corner and each face?

2) An atom has a radius of 165 pm and crystallizes in the body-centered cubic unit cell. What is the volume of the unit cell in cm^3?

3) A substance has a triple point at a temperature of 17 ∘C and a pressure of 3.2 atmospheres. In what states can the substance exist on the surface of Earth at sea level (open to the atmosphere)? Check all that apply.

a.	liquid
b.	solid
c.	gas

Answer 1

ANSWER 1) You can't work it out with the info given. First they are H^- ions
Corners H^- 8×⅛ = 1 Faces 6×½ =3 Total = 4 H^- Na = 4 to balance charges. But this is Z the number of formula units inside the cell. You are then meant to make the leap that NaH has a NaCl (that I already know) where the coordination is 6:6 and each ion is octahedrally coordinated.

ANSWER 2)

The triangle we will use runs differently than the triangle used in fcc calculations.
d is the edge of the unit cell, however d√2 is NOT an edge of the unit cell. It is a
diagonal of a face of the unit cell. 4r is a body diagonal. Since it is a right triangle,
the Pythagorean Theorem works just fine.

Using the Pythagorean Theorem, we find: d² + (d√2)² = (4r)²

3d² = (4r)²=16*165*165pm^2

d=381.05pm

cube root of volume = edge lenght (d)

volume= d^3=5.53*10^7pm^3= 5.53*10^-23cm^3

ANSWER 3)

In our case, the triple point is slightly to the left of one given in the standard figure and will be at Pressure of 3.2atm instead of 0.25 given in the figure. At sea level,the pressure will be 1atm=P(atmospheric). Hence,as per figure it can exist as a gas or solid but not liquid.