It is given that the isomolar solutions are sodium salt of Group 16 elements are present in equal molar concentration. The salts on hydrolysis with water, it produces an acid and base, as follows: I. Na2O+H2O⟶2NaOH II. Na2 S+H2O⟶2NaOH+H2 S III. Na2Se+H2O⟶2NaOH +H2Se IV. Na2Te+H2O>2NaOH +H2Te The acidic strength of the hydrides is in the order H2O<H2 S<H2Se< H2Te, due to increase in the atomic size. As the acidic character of the hydride increases down the group, the basic character of the salt decreases down the group. Therefore, the order of basic character of the salts will be: Na2O>Na2 S>Na2Se>Na2Te Therefore, the order of pH of the given aqueous solution is pH1>pH2> pH3>pH4 Related Theory The pH of the isomolar solution, can be determined with respect to the basic character of the sodium salts of the Group 16 elements. These salts undergo hydrolysis, forming hydrides, whose acidic character can account for the basic character of the salt, formed through neutralisation. A Caution In the reverse reaction of neutralisation, from the acidic strength of the hydrides, we can obtain the order of ease of neutralisation of the sodium hydroxide with the hydrides. As more the acidity of the hydride, more will be the neutralisation to form the salt.