Question

Find the number of proton donor acid(s) from the following:
$B{{(OH)}_{3}}$, $Mg{{(OH)}_{2}}$, $Si{{(OH)}_{4}}$, $S{{O}_{2}}{{(OH)}_{2}}$, $Ba{{(OH)}_{2}}$

Answer 1

According to the Bronsted-Lowry concept of acids and bases, any species donating proton (${{H}^{+}}$ ion) to other species are acids whereas Lewis concept defines acids as the species which can accept a pair of electrons from other molecules.

Complete answer:
Proton donor acids can donate one or more ${{H}^{+}}$ ions to other molecules (bases). When proton donor acids are dissolved in water, they dissociate to give ${{H}^{+}}$ ions. Hydration of protons by water molecules then forms hydronium ions.

& HA\to {{H}^{+}}+{{A}^{-}} \\\ & {{H}^{+}}+{{H}_{2}}O\to {{H}_{3}}{{O}^{+}} \\\ \end{aligned}$$ Now let us examine all the compounds given one by one to find out the total number of proton donor acids. \- $$B{{(OH)}_{3}}$$: Orthoboric acid or boric acid. It is a monobasic Lewis acid. it does release a proton but rather accepts a pair of electrons. It belongs to the class of aprotic acids. \- $Mg{{(OH)}_{2}}$: Magnesium hydroxide is not an acid but a base. It is a weaker base than alkali metal bases like NaOH, KOH, etc. $Mg{{(OH)}_{2}}$ is a base according to the Bronsted-Lowry concept of bases. When $Mg{{(OH)}_{2}}$ is dissolved in water, it dissociates into $M{{g}^{2+}}$and $O{{H}^{-}}$ ions. $Mg{{(OH)}_{2}}\to M{{g}^{2+}}(aq)+2O{{H}^{-}}$ \- $Si{{(OH)}_{4}}$: Silicic acid or orthosilicic acid is a weak acid. Although it is a weak acid it gives a proton and hence, is a proton donor acid. $Si{{(OH)}_{4}}+{{H}_{2}}O\to Si{{(OH)}_{3}}{{O}^{-}}+{{H}^{+}}$ \- $S{{O}_{2}}{{(OH)}_{2}}$: It is sulphuric acid i.e., ${{H}_{2}}S{{O}_{4}}$. Sulphuric acid is a very strong acid. It ionizes first into ${{H}^{+}}$and $HSO_{4}^{-}$. Then, ${{H}^{+}}$ ions are surrounded by water molecules to form hydronium ions. $\begin{aligned} & {{H}_{2}}S{{O}_{4}}\to {{H}^{+}}+HSO_{4}^{-} \\\ & {{H}^{+}}+{{H}_{2}}O\to {{H}_{3}}{{O}^{+}} \\\ \end{aligned}$ $HSO_{4}^{-}$ further dissociates to give ${{H}^{+}}$ and $SO_{4}^{2-}$ ions. The overall reaction of sulphuric acid in water is given as: $${{H}_{2}}S{{O}_{4}}+2{{H}_{2}}O\to 2{{H}_{3}}{{O}^{+}}+SO_{4}^{2-}$$ \- $$Ba{{(OH)}_{2}}$$ : Barium hydroxide solution is a strong base. It completely dissociates in water into $B{{a}^{2+}}$ and $$O{{H}^{-}}$$ ions. Hence, it is a Bronsted-Lowry base. $$Ba{{(OH)}_{2}}\to B{{a}^{2+}}+2O{{H}^{-}}$$ The proton donor acids are sulphuric acid ($S{{O}_{2}}{{(OH)}_{2}}$) and silicic acid ($Si{{(OH)}_{4}}$). Therefore, the number of proton donor acids is two. **Note:** Note that $$B{{(OH)}_{3}}$$ is also an acid but it is a Lewis acid. It is electron deficient in nature as boron has only six electrons in its outermost shell. Therefore, when dissolved in water, it accepts a pair of electrons from water in the form of $$O{{H}^{-}}$$ ion.