Question

Assuming equal concentrations, rank these solutions by pH.

Ca(OH)2(aq) LiOH(aq) NH3(aq) HBrO(aq) HBr(aq)

Answer 1

Concepts and reason

Write the dissociation reactions of the given acids and bases. Count the number of hydride and hydroxide ion given by each of the species.

Arrange the given species on the basis of number of hydride and hydroxide ions produced by the given bases and acids.

The species which are present in equilibrium are weak species.

Fundamentals

pH is defined as the negative logarithm of the hydrogen ion concentration. The scale of pH is from 0 to 14. The solutions in the pH range 0-6 are acidic in nature and the solutions in the pH range 8-14 are basic in nature. A solution with pH of 7 is said to be a neutral solution.

Strong acids and bases undergo complete dissociation in the solution. On dissociation of strong acids and bases, ${{\rm{H}}^ + }$ and ${\rm{O}}{{\rm{H}}^ - }$ are produced in the solution which are responsible for the acidity and basicity of the solution.

The solution which produces maximum ${{\rm{H}}^ + }$ is the most acidic solution.

The solution which produces maximum ${\rm{O}}{{\rm{H}}^ - }$ is the most basic solution.

The dissociation reaction of ${\rm{Ca}}{\left( {{\rm{OH}}} \right)_2}$ is written as follows:

${\rm{Ca}}{\left( {{\rm{OH}}} \right)_2} \to {\rm{C}}{{\rm{a}}^{2 + }} + 2{\rm{O}}{{\rm{H}}^ - }$

The dissociation reaction of ${\rm{LiOH}}$ is written as follows:

${\rm{LiOH}} \to {\rm{L}}{{\rm{i}}^ + } + {\rm{O}}{{\rm{H}}^ - }$

The dissociation reaction of ${\rm{N}}{{\rm{H}}_3}$ is written as follows:

${\rm{N}}{{\rm{H}}_3} + {{\rm{H}}_2}{\rm{O}} \leftrightarrow {\rm{N}}{{\rm{H}}_4}^ + + {\rm{O}}{{\rm{H}}^ - }$

The dissociation reaction of ${\rm{HBrO}}$ is written as follows:

${\rm{HBrO}} \leftrightarrow {{\rm{H}}^ + } + {\rm{Br}}{{\rm{O}}^ - }$

The dissociation reaction of ${\rm{HN}}{{\rm{O}}_3}$ is written as follows:

${\rm{HN}}{{\rm{O}}_{\rm{3}}} \to {{\rm{H}}^ + } + {\rm{N}}{{\rm{O}}_3}^ -$

Hence, the descending order with respect to pH is as follows:

${\rm{Ca}}{\left( {{\rm{OH}}} \right)_2} > {\rm{LiOH}} > {\rm{N}}{{\rm{H}}_3} > {\rm{HBrO}} > {\rm{HN}}{{\rm{O}}_3}$

Ans:

The descending order with respect to pH is as follows:

${\rm{Ca}}{\left( {{\rm{OH}}} \right)_2} > {\rm{LiOH}} > {\rm{N}}{{\rm{H}}_3} > {\rm{HBrO}} > {\rm{HN}}{{\rm{O}}_3}$