1. KCl has the rock salt structure witha = 6.2931 Å. Calculate the density of KCl.

There are 4 K+ ions and 4 Cl- ions in each unit cell, therefore

total mass of ions = 4 (Ar K) + 4 (Ar Cl)

= 4 (40.00 g/mol) + 4 (35.45 g/mol)

= 301.8 g/mol

1. KCl has the rock salt structure witha = 6.2931 Å. Calculate the density of KCl.

Total mass of ions = 301.8 g/mol

Then we divide it by Avogado’s number (NA),

total mass of ions = m = 5.013 x 10-22 g

The volume of unit cell is V = a3 = (6.2931 Å)3

Converting Å3 to cm3, the volume is given by

V = 249.22 Å3 x (10-10 x 102 cm/Å)3

= 2.4922 x 10-22 cm3

Finally, from the definition of density :

density = m / V

= (5.013 x 10-22 g) / (2.4922 x 10-22 cm3)

= 2.011 g/cm3

1. KCl has the rock salt structure witha = 6.2931 Å. Calculate the density of KCl.

The relation of formula unit (Z), density (d), empirical formula molar mass (Mr), and volume of each unit cell (V) can be expressed by

in which NA is Avogadro’s number.

The volume of unit cell is V = a3 = (3.97 Å)3

Converting Å3 to cm3, the volume is given by

V = 62.57 Å3 x (10-10 x 102 cm/Å)3

= 6.257 x 10-23 cm3

2. TlBr has a cubic unit cell with a = 3.97 Å. and density 7.458 g/cc. How many formula units, Z, are in the unit cell? Suggest the possible structure.

Molar mass of TlBr (Thallium bromide)

= Ar Tl + Ar Br

= (204.38 + 79.90) g/mol

= 284.28 g/mol

From the equation

we can find the value of Z by rearrange this equation into

2. TlBr has a cubic unit cell with a = 3.97 Å. and density 7.458 g/cc. How many formula units, Z, are in the unit cell? Suggest the possible structure.

2. TlBr has a cubic unit cell with a = 3.97 Å. and density 7.458 g/cc. How many formula units, Z, are in the unit cell? Suggest the possible structure.

Z = 0.988 ≈ 1

In conclusion, there is one TlBr formula unit per unit cell. There are one Tl atom and one Br atom in each unit cell. Therefore, the most possible structure of TlBr is CsCl type of ionic solid (BCC substitutional).

23 237.458 g/cc 6.257 10 cc 6.02 10 /mol

(284.28 g/ )

( ) ( )

molZ

2. TlBr has a cubic unit cell with a = 3.97 Å. and density 7.458 g/cc. How many formula units, Z, are in the unit cell? Suggest the possible structure.

2. TlBr has a cubic unit cell with a = 3.97 Å. and density 7.458 g/cc. How many formula units, Z, are in the unit cell? Suggest the possible structure.