Question

KF crystallises in an NaCl type structure and the density of the crystal was found to be 2.48 g cm–3
. Which of the following is correct? (K = 39 g/mol and F = 19 g/mol) (use 3 1248 10.76 
) (use NA = 6 × 1023)
(1) The number of nearest neighbour of
F

is 12
(2) The nearest neighbour of F

is present at a distance of 269 pm (approx.)
(3) The distance between two F

at the adjacent corner is 10.76 Å
(4) The distance between the F

ions cannot be calculated without the radius ratio of K+ and F

Answer 1

Answer:

• please mark as brillint

Answer 2

The correct question: KF crystallizes in an NaCl type structure and the density of the crystal was found to be 2.48 g cm⁻³. Which of the following is correct?

(K = 39 g/mol and F = 19 g/mol) (use $\sqrt[3]{1248}$ = 10.76) (use $N_{A}$ = 6.022 x $10^{23}$)

a). The number of nearest neighbors of F⁻ is 12.

b). The nearest neighbor of F⁻ is present at a distance of 269 pm (approx.).

c). The distance between two F at the adjacent corner is 10.76 Å.

d).The distance between the F⁻ ions cannot be calculated without the radius ratio of K and F.

The correct answers is option (b). The nearest neighbor of F⁻ is present at a distance of 269 pm (approx.)

• The distance between K⁺ and F⁻ = 2.687 Aº = 268.7pm (approximately 269 pm). Therefore option (b) is correct.

• The structure of NaCl is formed by repeating the face-centred cubic unit cell. KF crystal also has same structure. Hence, n = 4. Therefore option (a) is wrong.

• Density of KF crystal, d = 2.489 cm⁻³. Using the equation for density,  $2.48 = \frac{4 × 58}{a^{3} × 6.022 × 10^{23} }$. The Value of "a" (the distance between two F at the adjacent corner) can be found out using the above equation as: a = 5.37 x 10⁻⁸ cm = 5.37 Aº (approx.). Therefore the option  (c) is also incorrect.

• The distance between the F⁻ ions can be calculated without the radius ratio of K and F. Hence option (d) is also incorrect.

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