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# A substance has density of 2 kg $dm^{-3}$ and it crystallizes to fcc lattice with edge-length equal to 700pm, then the molar mass of the substance is

$\rho=\large\frac{n\times M_m}{N_A\times a^3}$
$2=\large\frac{4\times M_m}{6.023\times 10^{23}\times (7\times 10^{-8})^3}$
(since, effective number of atoms in unit cell = 4)
On solving we get $M_m=103.03gm/mol$
Hence (B) is the correct answer.