The $pH$ of milk,black coffee,tomato juice,lemon juice and egg white are $6.8, 5.0, 4.2, 2.2$ and $7.8$ respectively. Calculate the corresponding hydrogen ion concentration $[H^+]$ in each.

The hydrogen ion concentration $[H^+]$ is calculated using the formula: $pH = -\log [H^+]$,which implies $[H^+] = 10^{-pH}$.
$(i)$ For milk $(pH = 6.8)$: $[H^+] = 10^{-6.8} = 1.58 \times 10^{-7} \, M$
$(ii)$ For black coffee $(pH = 5.0)$: $[H^+] = 10^{-5.0} = 1.0 \times 10^{-5} \, M$
$(iii)$ For tomato juice $(pH = 4.2)$: $[H^+] = 10^{-4.2} = 6.31 \times 10^{-5} \, M$
$(iv)$ For lemon juice $(pH = 2.2)$: $[H^+] = 10^{-2.2} = 6.31 \times 10^{-3} \, M$
$(v)$ For egg white $(pH = 7.8)$: $[H^+] = 10^{-7.8} = 1.58 \times 10^{-8} \, M$