Question

If you have to dissolve lead (II) oxide $\left( {{\rm{PbO}}} \right)$ , which acid will you choose?
A. ${{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}}$
B. ${\rm{HN}}{{\rm{O}}_{\rm{3}}}$
C. ${{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}}$
D. ${\rm{HCl}}$

Answer 1

Dissolution of a given substance in a suitable solvent would give a clear solution so we can rule out those acids that will give a precipitate with lead.

Complete answer
We have seen the use of precipitation reactions in qualitative analysis of salts. Formation of a precipitate can be observed easily and predictions can be made for cations and anions. We know that the theoretical bases for the involved tests are solubility product and common ion effect. By adding a reagent, we increase the concentration of a particular ion and see if it gets precipitated confirming its presence. For ${\rm{P}}{{\rm{b}}^{{\rm{2 + }}}}$ ions, we have seen different precipitation reactions while performing qualitative test. While doing the preliminary test, aqueous solution containing lead ions give a white precipitate upon addition of dilute ${\rm{HCl}}$ that can be shown as follows:
${\rm{P}}{{\rm{b}}^{{\rm{2 + }}}} + {\rm{C}}{{\rm{l}}^ - } \to {\rm{PbC}}{{\rm{l}}_{\rm{2}}} \downarrow$
This precipitate is soluble in hot water but when we add few drops of alcohol and dilute ${{\rm{H}}_{\rm{2}}}{\rm{S}}{{\rm{O}}_{\rm{4}}}$ again we get a white precipitate as follows:
${\rm{P}}{{\rm{b}}^{{\rm{2 + }}}} + {\rm{SO}}_4^{2 - } \to {\rm{PbS}}{{\rm{O}}_{\rm{4}}} \downarrow$
However, precipitate of lead sulfide dissolves in ${\rm{HN}}{{\rm{O}}_{\rm{3}}}$ as lead nitrate is formed which we can show by the following equation:
${\rm{P}}{{\rm{b}}^{{\rm{2 + }}}} + {\rm{NO}}_3^ - \to {\rm{Pb}}{\left( {{\rm{N}}{{\rm{O}}_{\rm{3}}}} \right)_2}$
Similarly, we can write the equation for precipitation reaction of lead with ${{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}}$ as follows:
${\rm{P}}{{\rm{b}}^{{\rm{2 + }}}} + {\rm{PO}}_4^{3 - } \to {\rm{P}}{{\rm{b}}_3}{\left( {{\rm{P}}{{\rm{O}}_{\rm{4}}}} \right)_2} \downarrow$
From the above reactions, we can conclude that it is only in ${\rm{HN}}{{\rm{O}}_{\rm{3}}}$acid that ${\rm{PbO}}$ can be dissolved as in all other cases it forms an insoluble precipitate.

**Hence, the correct option is B.

Note:**
We know ${\rm{PbO}}$ is an amphoteric oxide so it can be dissolved in acids and bases but not in all of them as it reacts differently upon reacting with different acids.