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Answer:
The theoretical density for Niobium is
.
Explanation:
Formula used :

where,
= density of the unit cell
Z = number of atom in unit cell
M = atomic mass
= Avogadro's number
a = edge length of unit cell
We have :
Z = 2 (BCC)
M = 92.91 g/mol ( Niobium)
Atomic radius for niobium = r = 0.143 nm
Edge length of the unit cell = a
r = 0.866 a (BCC unit cell)


On substituting all the given values , we will get the value of 'a'.


The theoretical density for Niobium is
.
Solution :


Using the data table for refrigerant-134a at P = 120 psia



∴ 
For pressure, P = 20 psia





Change in temperature, 


Now we find the quality,



The final energy,



Change in internal energy

= 38.09297-40.5485
= -2.4556