Question

Why is the H2SO4 unable to accept a proton?

Correct answer: HS- NH3 Amphiprotic species are able to both accept and donate protons. According to the Brønsted-Lowry definitions of acids and bases, amphiprotic species can act as both acids and bases. H2SO4 will donate, but not accept a proton. PO will accept a proton, but has no proton to donate.

Answer 1

According to Bronsted-Lowry acid-base definitions, acids donate proton to other substance(s) and bases accept proton from acidic substance(s). Now H​​​​​​₂SO₄  is a strong dibasic acid(can donate two protons), and cannot accept protons from substances that are less acidic than it.

[H​₂SO₄ can accept proton from HClO₄ , which is a stronger acid than H​​​​​​₂SO₄.]

Amphoteric species can both accept and donate proton(s). Here HS^- , is an amphoteric species. It can donate a proton, forming S^2-​ and can also accept a proton, forming H_{​​​​​2}S.

NH_{​​​​​3} is a base. It can only accept proton.

PO₄^3- have no protons to donate, thus only can accept protons. It is a base.