Given:

t\(_c\) = ?  V\(_c\) = 60cm\(^3\) ,  M\(_c\) = 35.5 x 2 = 71

t\(_o\) = 7s; V\(_o\) = 30cm\(^3\) ,  M\(_o\) = 16 x 2 = 32

According to Graham's law of diffusion, R ∝  1/√M

    \(\frac{R_o}{R_c}\) = \(\sqrt{\frac{M_c}{M_o}}\)

 Also, Rate of diffusion =  \(\frac{Volume}{time}\)

\(\frac{R_o}{R_c}\) =  \(\frac{V_o}{t_o}\) ÷ \(\frac{V_c}{t_c}\) = \(\frac{V_o}{t_o}\) X \(\frac{t_c}{V_c}\)

Therefore, \(\frac{V_o}{t_o}\) X \(\frac{t_c}{V_c}\) = \(\sqrt{\frac{M_c}{M_o}}\)

 \(\frac{{30}\times{t_o}}{{7}{60}}\) = \(\sqrt{\frac{71}{32}}\)

 \(\frac{t_c}{14}\) = 1.489

 t\(_c\) = 14 x 1.489

 t\(_c\) = 20.846

 t\(_c\) = 21s