1) A crystalline solid of a pure substance has a face-centered 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 256 g of the crystal is N × 1024. The value of N is A) $2\times 10^{26}$ B) $4\times 10^{26}$ C) $2\times 10^{24}$ D) $4\times 10^{26}$ Answer: Option CExplanation:Density (ρ )= $8=\frac{4\times M}{N_{A}(4\times 10^{-8}cm)^{3}}$ M= 128 × 10-24 NA $\Rightarrow$ No of atoms= $\frac{256}{M}\times N_{A}$ = $\frac{256}{128\times 10^{-24}N_{A}}\times N_{A}$ =2 × 1024
