The physiological reactions occurring in the human body take place in the range of $......\,pH.$

The $pH$ of a water sample collected from a river was found to be acidic in the range of $3.5 - 4.5$. On the banks of the river were several factories that were discharging effluents into the river. The effluents of which one of the following factories is the most likely cause for lowering the $pH$ of river water?

Which of the following statements is correct about an aqueous solution of an acid and of a base?
$(i)$ Higher the $pH$,stronger the acid
$(ii)$ Higher the $pH$,weaker the acid
$(iii)$ Lower the $pH$,stronger the base
$(iv)$ Lower the $pH$,weaker the base

What is the concentration of $[H_3O^+]$ in distilled water at $298 \; K$?

How is an aqueous solution with $pH = 2$ compared to an aqueous solution with $pH = 4$?

Which of the following gives the correct increasing order of acidic strength?