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Question
An element crystallises in a b.c.c lattice with cell edge of 500 pm. The density of the element is 7.5g cm-3. How many atoms are present in 300 g of the element?
Solution
a = 500 pm = 500 x 10-10 cm
z = 2
m = 300 g
`Density(d)=(zM)/(a^2N_A)`
`7.5=(2xxm)/((500)^3xx10^(-30)xx6.02xx10^23)`
`M=(7.5xx(500)^3xx10(-30)xx6.02xx10^23)/2`
`M=282.18 " g/mol"`
`"Molar mass (M)"=("Mass of compound"xxN_A)/"Number of atoms"`
`282.18=(300xx6.02xx10^23)/"Number of atoms"`
Number of atoms = 6.4 x 1023
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