What transition in the hydrogen spectrum would have the same wavelength as the Balmer transition $\mathrm{n}=4$ to $\mathrm{n}=2$ of ${\mathrm{He}}^{+}$ spectrum ?

Find (a) the total number and (b) the total mass of neutrons in $7 \mathrm{mg} \mathrm{of}$ ${}^{14}\mathrm{C}$. 

(Assume that mass of a neutron= $1.675\times {10}^{-27} \mathrm{kg}$)

In Rutherford’s experiment, generally the thin foil of heavy atoms, like gold, platinum etc. have been used to be bombarded by the α-particles. If the thin foil of light atoms like Aluminium etc. is used, what difference would be observed from the above results ?