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(i) What is ferromagnetic substance? (...

(i) What is ferromagnetic substance?
(ii) KBr crystallises in face-centred cubic (fcc) crystals. The density and formula mass of KBr crystal are `2.65g*cm^(-3)` and `119g*mol^(-1)` respectively. Find the distance between `K^(+)andBr^(-)` ions in KBr crystal.
or, (i) Which kind of defect in ionic crystal does not alter the density?
(ii) An element X has atomic mass `60g*mol^(-1)` and density `6.23g*cm^(-3)`. The edge length of its unit cell is 400 pm. Identify the type of the unit cubic cell. Calculate the radius of X atom.

Text Solution

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(ii) For cubic unit cell, we know,
`rho=(ZxxM)/(Nxxa^(3))or,2.65=(4xx119)/(6.022xx10^(23)xxa^(3))`
or, a = 6.68Å
Therefore, the distance between `K^(+)andBr^(-)` ion = `6.68/2Å`
= 3.34Å.
or, (i) In Frenkel defect, the density of ionic crystal does not alter.
(ii) We know, `rho=(ZxxM)/(Nxxa^(3))`
`therefore6.23=(Zxx60)/(6.022xx10^(23)xx(400xx10^(-10))^(3))` or, Z = 4
Z = 4 suggests that the uniit cell is face-centred cubic.
`therefore` Radius of the element,
`r=a/(2sqrt2)=400/(2sqrt2)=141.42`pm
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