
Concept explainers
(a)
Interpretation:
The spacing between the layers that is present in the potassium iodide crystal that gives rise to 14.7 degrees of diffraction has to be calculated.
(a)

Answer to Problem 11.88QE
The spacing between the layers in crystal of KI is 3.526×10−10 m.
Explanation of Solution
Wavelength of X-ray is given as 1.790×10−10 m. The angle of diffraction is given as 14.7 degrees.
Bragg equation can be used to determine the spacing between the layers. Bragg equation is given as shown below;
d=nλ2sinθ (1)
Where,
n is given as 1.
d is the distance between layers.
λ is the wavelength of X-ray.
θ is the angle of diffraction.
Substituting the values in equation (1), the spacing between the layers can be calculated as shown below;
d=1×1.790×10−10 m2(sin14.7°)=1.790×10−10 m2(0.2538)=3.526×10−10 m
Therefore, the spacing between layers is 3.526×10−10 m.
(b)
Interpretation:
Density of KI has to be calculated.
(b)

Answer to Problem 11.88QE
Density of potassium iodide is 3.144 g/cm3.
Explanation of Solution
The crystal is of KI. The spacing between the layers is 3.526×10−10 m. The edge length of the cube is given as twice of the length of spacing between the layers. Therefore, the edge length is calculated as follows;
Edge length=2×3.526×10−10 m=7.052×10−10 m
The unit cell of potassium iodide is face centered cubic cell. Therefore, there are eight corners and thus one atom is shared by eight other unit cells that are present. One atom is present at the center of each face of the cell. Therefore, the total
6 faces×1 atom2 faces+8 corners×1 atom8 corners = 4 atoms
The volume of the cubic cell can be calculated using the edge length. This can be done as shown below;
Volume=(edge length)3=(7.052×10−10 m)3=350.70×10−30 m3
Volume of cubic cell can be converted into centimeters as shown below using conversion factors;
350.70×10−30 m3=350.70×10−30 m3×100 cm31 m3=3.507×10−22 cm3
Molar mass of KI is 166.00 g/mol. Avogadro’s number is 6.022×1023 atoms. Therefore, the mass of KI can be calculated as shown below;
Mass of crystal=number of atoms×1Avogadro's number× molar mass= 4×16.022×1023 atoms×166.00 g/mol= 110.26×10-23 g
Therefore, mass of KI crystal is 110.26×10-23 g.
Density of the potassium iodide crystal can be calculated using the density formula as follows;
Density=MassVolume=110.26×10-23 g3.507×10−22 cm3=31.44×10−1 g/cm3=3.144 g/cm3
Thus, density of potassium iodide is calculated as 3.144 g/cm3.
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Chapter 11 Solutions
Chemistry: Principles and Practice
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