$\mathrm{NaNO}_3$ is salt formed by the combination of a strong acid $\mathrm{HNO}_3$ and a strong base $\mathrm{NaOH}$ and hence it is neutral with a $\mathrm{pH}$.
$\mathrm{CH}_3 \mathrm{COOK}$ is formed by the combination of weak acid $\mathrm{CH}_3 \mathrm{COOH}$ and strong base $\mathrm{KOH}$ and hence $\mathrm{pH}$ will be above 7 .
$\mathrm{NH}_4 \mathrm{Cl}$ is formed by the formation of strong and $\mathrm{HCl}$ and a weak base $\mathrm{NH}_4 \mathrm{OH}$ and hence $\mathrm{pH}$ will be lesser than 7.
$\mathrm{Na}_2 \mathrm{CO}_3$ is a salt formed by the strong base $\mathrm{NaOH}$ and weak acid $\mathrm{H}_2 \mathrm{CO}_3$. Hence, $\mathrm{pH}$ of the salt will be highest.
The pH scale was invented in 1909 by a Danish biochemist Soren Sorensen. It describes how many hydrogen ions (protons) are present in a solution: the higher the pH, the lower the hydrogen ion concentration, and vice versa. But the scale does not have fixed limits, so it is indeed possible to have a pH above 14 or below zero. For example, concentrated hydrochloric acid can have a pH of around -1 , while sodium hydroxide solution can have a pH as high as 15 .