1.

The pyknometric density of `NaCl` crystal is `2.165 xx 10^(3 kg m^(-3)` while its `X`-ray density is `2.178 xx 10^(-3) kg m^(-3)`. The fraction of unoccupied sites in `NaCl` crystal is a. `5.96` b. `5.96 xx 10^(-2)` c. `5.96 xx 10^(-1)` d. `5.96 xx 10^(-3)`

Answer» d. Molar volume from pyknometric density
`= ("Weight or mass")/("Volume") = ("Weight" (W))/(2.165 xx 10^(3))m^(3)`
Molar volume from `X`-ray density `= (W)/(2.178 xx 10^(3))m^(3)`
Volume unoccupied `= (W)/(10^(3)) ((1)/(2.165) - (1)/(2.178))m^(3)`
`= (0.013 W xx 10^(-3))/(2.165 xx 2.178)`
`:.` Fraction unoccupied `= ((0.013 W xx 10^(-3))/(2.165 xx 2.178))//((W xx 10^(23))/(2.165))`
`= 5.96 xx 10^(-3)`


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