# 14.3 Relative strengths of acids and bases  (Page 11/18)

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Rank the compounds in each of the following groups in order of increasing acidity or basicity, as indicated, and explain the order you assign.

(a) acidity: NaHSO 3 , NaHSeO 3 , NaHSO 4

(b) basicity: ${\text{BrO}}_{2}{}^{\text{−}},$ ${\text{ClO}}_{2}{}^{\text{−}},$ ${\text{IO}}_{2}{}^{\text{−}}$

(c) acidity: HOCl, HOBr, HOI

(d) acidity: HOCl, HOClO, HOClO 2 , HOClO 3

(e) basicity: ${\text{NH}}_{2}{}^{\text{−}},$ HS , HTe , ${\text{PH}}_{2}{}^{\text{−}}$

(f) basicity: BrO , ${\text{BrO}}_{2}{}^{\text{−}},$ ${\text{BrO}}_{3}{}^{\text{−}},$ ${\text{BrO}}_{4}{}^{\text{−}}$

(a) NaHSeO 3 <NaHSO 3 <NaHSO 4 ; in polyoxy acids, the more electronegative central element—S, in this case—forms the stronger acid. The larger number of oxygen atoms on the central atom (giving it a higher oxidation state) also creates a greater release of hydrogen atoms, resulting in a stronger acid. As a salt, the acidity increases in the same manner. (b) ${\text{ClO}}_{2}{}^{\text{−}}<{\text{BrO}}_{2}{}^{\text{−}}<{\text{IO}}_{2}{}^{\text{−}};$ the basicity of the anions in a series of acids will be the opposite of the acidity in their oxyacids. The acidity increases as the electronegativity of the central atom increases. Cl is more electronegative than Br, and I is the least electronegative of the three. (c) HOI<HOBr<HOCl; in a series of the same form of oxyacids, the acidity increases as the electronegativity of the central atom increases. Cl is more electronegative than Br, and I is the least electronegative of the three. (d) HOCl<HOClO<HOClO 2 <HOClO 3 ; in a series of oxyacids of the same central element, the acidity increases as the number of oxygen atoms increases (or as the oxidation state of the central atom increases). (e) ${\text{HTe}}^{\text{−}}<{\text{HS}}^{\text{−}}<<\phantom{\rule{0.2em}{0ex}}{\text{PH}}_{2}{}^{\text{−}}<{\text{NH}}_{2}{}^{\text{−}};$ ${\text{PH}}_{2}{}^{\text{−}}$ and ${\text{NH}}_{2}{}^{\text{−}}$ are anions of weak bases, so they act as strong bases toward H + . ${\text{HTe}}^{\text{−}}$ and HS are anions of weak acids, so they have less basic character. In a periodic group, the more electronegative element has the more basic anion. (f) ${\text{BrO}}_{4}{}^{\text{−}}<{\text{BrO}}_{3}{}^{\text{−}}<{\text{BrO}}_{2}{}^{\text{−}}<{\text{BrO}}^{\text{−}};$ with a larger number of oxygen atoms (that is, as the oxidation state of the central ion increases), the corresponding acid becomes more acidic and the anion consequently less basic.

Both HF and HCN ionize in water to a limited extent. Which of the conjugate bases, F or CN , is the stronger base? See [link] .

The active ingredient formed by aspirin in the body is salicylic acid, C 6 H 4 OH(CO 2 H). The carboxyl group (−CO 2 H) acts as a weak acid. The phenol group (an OH group bonded to an aromatic ring) also acts as an acid but a much weaker acid. List, in order of descending concentration, all of the ionic and molecular species present in a 0.001- M aqueous solution of C 6 H 4 OH(CO 2 H).

$\left[{\text{H}}_{2}\text{O}\right]\phantom{\rule{0.2em}{0ex}}>\left[{\text{C}}_{6}{\text{H}}_{4}\text{OH}\left({\text{CO}}_{2}\text{H}\right)\right]\phantom{\rule{0.2em}{0ex}}>{\text{[H}}^{\text{+}}\text{]}\phantom{\rule{0.2em}{0ex}}\text{0}>{\text{[C}}_{6}{\text{H}}_{4}\text{OH}{\left({\text{CO}}_{2}\right)}^{\text{−}}\right]\gg \left[{\text{C}}_{6}{\text{H}}_{4}\text{O}{\left({\text{CO}}_{2}\text{H}\right)}^{\text{−}}\right]\phantom{\rule{0.2em}{0ex}}>\left[{\text{OH}}^{\text{−}}\right]$

What do we represent when we write:
${\text{CH}}_{3}{\text{CO}}_{2}\text{H}\left(aq\right)+{\text{H}}_{2}\text{O}\left(l\right)\phantom{\rule{0.2em}{0ex}}⇌\phantom{\rule{0.2em}{0ex}}{\text{H}}_{3}{\text{O}}^{\text{+}}\left(aq\right)+{\text{CH}}_{3}{\text{CO}}_{2}{}^{\text{−}}\left(aq\right)?$

Explain why equilibrium calculations are not necessary to determine ionic concentrations in solutions of certain strong electrolytes such as NaOH and HCl. Under what conditions are equilibrium calculations necessary as part of the determination of the concentrations of all ions of some other strong electrolytes in solution?

Strong electrolytes are 100% ionized, and, as long as the component ions are neither weak acids nor weak bases, the ionic species present result from the dissociation of the strong electrolyte. Equilibrium calculations are necessary when one (or more) of the ions is a weak acid or a weak base.

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