Q.ii. What is meant by conjugate acid-base pair?

Acids and bases can be defined by three progressively more general theories. The Arrhenius theory defines an acid as a substance that gives H+ ions in water and a base as a substance that gives OH- ions in water, but it fails for bases like NH3 or Na2CO3 that have no OH group. The Bronsted-Lowry theory instead defines an acid as a proton (H+) donor and a base as a proton acceptor -- so a reaction such as $HCl+H_2O\rightleftharpoons H_3O^++Cl^-$ pairs HCl with its conjugate base Cl-, and H2O with its conjugate acid H3O+ (a conjugate acid-base pair differs by exactly one proton). Water itself is amphoteric under this theory -- it acts as a base towards HCl and as an acid towards NH3. The Lewis theory is the broadest: a Lewis acid accepts a share in an electron pair, a Lewis base donates one, so species with no proton at all to give or take (a bare metal cation, or a molecule like AlCl3 with an incomplete octet) can still be acids by accepting an electron pair from a donor such as NH3. Acids/bases are separately classed as strong (almost completely ionized: HCl, HNO3, H2SO4, NaOH, KOH) or weak (only partially ionized: CH3COOH, HF, H2S, NH4OH), based on how far this ionization proceeds -- this strong/weak split, not the acid-base theory used, is what determines the substance's dissociation constant and its behaviour in every other concept in this chapter.