Because acetic acid is a stronger acid than water, it must also be a weaker base, with a lesser tendency to accept a proton than \(H_2O\). Measurements of the conductivity of 0.1 M solutions of both HI and \(HNO_3\) in acetic acid show that HI is completely dissociated, but \(HNO_3\) is only partially dissociated and behaves like a weak acid in this solvent. This result clearly tells us that HI is a stronger acid than \(HNO_3\). The relative order of acid strengths and approximate \(K_a\) and \(pK_a\) values for the strong acids at the top of Table \(\PageIndex<1>\) were determined using measurements like this and different nonaqueous solvents.
Into the aqueous selection, \(H_3O^+\) is the most powerful acidic and \(OH^?\) ‘s the most effective base that may can be found from inside the equilibrium which have \(H_2O\).
The leveling effect applies to solutions of strong bases as well: In aqueous solution, 
any base stronger than OH? is leveled to the strength of OH? because OH? is the strongest base that can exist in equilibrium with water. Salts such as \(K_2O\), \(NaOCH_3\) (sodium methoxide), and \(NaNH_2\) (sodamide, or sodium amide), whose anions are the conjugate bases of species that would lie below water in Table \(\PageIndex<2>\), are all strong bases that react essentially completely (and often violently) with water, accepting a proton to give a solution of \(OH^?\) and the corresponding cation:
Polyprotic Acids and you may Bases
As you learned, polyprotic acids instance \(H_2SO_4\), \(H_3PO_4\), and you will \(H_2CO_3\) contain much more than one ionizable proton, plus the protons was forgotten for the an excellent stepwise style. The brand new fully protonated species is always the most powerful acidic because it is easier to eradicate a beneficial proton of a simple molecule than simply from a adversely energized ion. Thus acidic energy minimizes for the loss of subsequent protons, and you will, correspondingly, brand new \(pK_a\) grows. Believe \(H_2SO_4\), for example:
The hydrogen sulfate ion (\(HSO_4^?\)) is both the conjugate base of \(H_2SO_4\) and the conjugate acid of \(SO_4^<2?>\). Just like water, HSO4? can therefore act as either an acid or a base, depending on whether the other reactant is a stronger acid or a stronger base. Conversely, the sulfate ion (\(SO_4^<2?>\)) is a polyprotic base that is capable of accepting two protons in a stepwise manner:
In contrast, in the second reaction, appreciable quantities of both \(HSO_4^?\) and \(SO_4^<2?>\) are present at equilibrium
Like any other conjugate acidbase pair, the strengths of the conjugate acids and bases are related by \(pK_a\) + \(pK_b\) = pKw. Consider, for example, the \(HSO_4^?/ SO_4^<2?>\) conjugate acidbase pair. From Table \(\PageIndex<1>\), we see that the \(pK_a\) of \(HSO_4^?\) is 1.99. Hence the \(pK_b\) of \(SO_4^<2?>\) is ? 1.99 = . Thus sulfate is a rather weak base, whereas \(OH^?\) is a strong base, so the equilibrium shown in Equation \(\ref<16.6>\) lies to the left. The \(HSO_4^?\) ion is also a very weak base (\(pK_a\) of \(H_2SO_4\) = 2.0, \(pK_b\) of \(HSO_4^? = 14 ? (?2.0) = 16\)), which is consistent with what we expect for the conjugate base of a strong acid.
- \(NH^+_<4(aq)>+PO^<3?>_ <4(aq)>\rightleftharpoons NH_<3(aq)>+HPO^<2?>_<4(aq)>\)
- \(CH_3CH_2CO_2H_<(aq)>+CN^?_ <(aq)>\rightleftharpoons CH_3CH_2CO^?_<2(aq)>+HCN_<(aq)>\)
Identify the conjugate acidbase pairs in each reaction. Then refer to Tables \(\PageIndex<1>\)and\(\PageIndex<2>\) and Figure \(\PageIndex<2>\) to determine which is the stronger acid and base. Equilibrium always favors the formation of the weaker acidbase pair.
The conjugate acidbase pairs are \(NH_4^+/NH_3\) and \(HPO_4^<2?>/PO_4^<3?>\). According to Tables \(\PageIndex<1>\) and \(\PageIndex<2>\), \(NH_4^+\) is a stronger acid (\(pK_a = 9.25\)) than \(HPO_4^<2?>\) (pKa = ), and \(PO_4^<3?>\) is a stronger base (\(pK_b = 1.68\)) than \(NH_3\) (\(pK_b = 4.75\)). The equilibrium will therefore lie to the right, favoring the formation of the weaker acidbase pair: