Specific volume ( vol. of 1 g) cylindrical virus particle

= 6.02 $\times$ 10^-2 cc/g

Radius of virus, r = 7 Å = 7 $\times$ 10^-8 cm

Volume of virus = $\pi {r^2}l$

= ${{22} \over 7} \times (7 \times {10^{ - 8}}) \times 10 \times {10^{ - 8}}$

= 154 $\times$ 10^-23 cc

wt. of one virus particle = ${{Volume} \over {Specific\,\,Volume}}$

$\Rightarrow$ ${{1.54 \times {{10}^{ - 23}}} \over {6.02 \times {{10}^{ - 2}}}}g$

$\therefore$ Molecular wt of virus = wt. of N_A particle

= ${{1.54 \times {{10}^{ - 23}}} \over {6.02 \times {{10}^{ - 2}}}} \times 6.02 \times 10^{-23} \,\,g/mol$

= 15400 g/mol = 15.4 kg/mol