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A crystalline solid of a pure substance ...

A crystalline solid of a pure substance has a face-centred cubic structure with a cell edge of 400 pm. If the density of the substance in the crystal is `8 g cm^(-3)`, then the number of atoms present in 256g of the crystal is `N xx 10^(24)`. The value of `N` is

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a = 400 pm `" " `, `" " d = 8 g cm^(-3) " " , " " w = 256 g " ", " " d = (z xx M_(0))/(a^(3) xx N_(A))`
`M_(0) = (d xx a^(3) N_(A))/( z ) = ( 8xx (400 xx 10^(-10))^(3) xx 6. 023 xx 10^(23))/(4) = 77.09 ` g/mol
Number of atoms = `(256)/(77.09) xx 6.023 xx 10^(23) = N xx 10^(24) " " rArr 2 xx 10^(24) = N xx 10^(24)` n = 2
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