Question

The order of pH of $0.200\text{ }M$ solutions of $\text{N}{{\text{H}}_{4}}\text{N}{{\text{O}}_{3}}\text{, NaN}{{\text{O}}_{3}}\text{, and N}{{\text{a}}_{2}}\text{C}{{\text{O}}_{3}}$ is:
A ) $\text{N}{{\text{H}}_{4}}\text{N}{{\text{O}}_{3}}<\text{ N}{{\text{a}}_{2}}\text{C}{{\text{O}}_{3}}<\text{ NaN}{{\text{O}}_{3}}$
B ) $\text{N}{{\text{H}}_{4}}\text{N}{{\text{O}}_{3}}<\text{ NaN}{{\text{O}}_{3}}<\text{ N}{{\text{a}}_{2}}\text{C}{{\text{O}}_{3}}$
C ) $\text{N}{{\text{a}}_{2}}\text{C}{{\text{O}}_{3}}<\text{ NaN}{{\text{O}}_{3}}<\text{ N}{{\text{H}}_{4}}\text{N}{{\text{O}}_{3}}$
D ) $\text{NaN}{{\text{O}}_{3}}<\text{ N}{{\text{H}}_{4}}\text{N}{{\text{O}}_{3}}<\text{ N}{{\text{a}}_{2}}\text{C}{{\text{O}}_{3}}$

Answer 1

pH is the negative logarithm of hydrogen ion concentration, with increase in the hydrogen ion concentration, the pH value decreases.

Complete step by step answer:
During hydrolysis of salt, either cation, or anion of both ions of salt, react with water to produce either hydronium ions or hydroxide ions. Salts can be classified into salt of strong acid with strong base, salt of strong acid with weak base, salt of weak acid with strong base and salt of weak acid with weak base. Each of the above types of salts gives different ions (hydronium ions or hydroxide ions) in solution. Based on these ions (hydronium ions or hydroxide ions), salt solutions can be classified into acidic, basic or neutral.
Consider hydrolysis of different types of salts in aqueous medium.
Greater is the hydrogen ion concentration of the solution, lower is its pH. Thus, more acidic solutions have lower pH than less acidic solutions. Greater is the hydroxide ion concentration of the solution, greater is its pH. Thus, more basic solutions have greater pH than less basic solutions.

$\text{NaN}{{\text{O}}_{3}}$ is a salt of strong acid (nitric acid) with a strong base (sodium hydroxide). Such salts do not undergo hydrolysis in aqueous medium. For such salts, $\left[ {{\text{H}}^{+}} \right]\text{ = }\left[ \text{O}{{\text{H}}^{-}} \right]\text{ = 1}{{\text{0}}^{-7}}\text{ M}$ and pH is $7$ .
$\text{N}{{\text{H}}_{4}}\text{N}{{\text{O}}_{3}}$ is a salt of strong acid (nitric acid) with weak base (ammonia). In aqueous solution, the cation (ammonium ion) undergoes hydrolysis to produce hydronium ions.

$\text{NH}_{4}^{+}\text{ + }{{\text{H}}_{2}}\text{O }\to \text{ N}{{\text{H}}_{3}}\text{ + }{{\text{H}}_{3}}{{\text{O}}^{+}}$
Hence, the solution is acidic with $pH\text{ }<\text{ }7$ .

Hence, the increasing order of pH is $\text{N}{{\text{H}}_{4}}\text{N}{{\text{O}}_{3}}<\text{ NaN}{{\text{O}}_{3}}\text{ }$.

$\text{N}{{\text{a}}_{2}}\text{C}{{\text{O}}_{3}}$ is a salt of weak acid (carbonic acid) with a strong base (sodium hydroxide). In aqueous solution, the anion (carbonate ion) undergoes hydrolysis to produce hydroxide ions.

${\text{CO}_{3}^{2+}\text{ + }{{\text{H}}_{2}}\text{O }\to \text{ HCO}_{3}^{-}\text{ + O}{{\text{H}}^{-}}}$
${\text{HCO}_{3}^{-}\text{ + }{{\text{H}}_{2}}\text{O }\to \text{ }{{\text{H}}_{2}}\text{C}{{\text{O}}_{3}}\text{ + O}{{\text{H}}^{-}}}$
Hence, the solution is basic with $pH\text{ }7$.
Hence, the increasing order of pH is $\text{N}{{\text{H}}_{4}}\text{N}{{\text{O}}_{3}}<\text{ NaN}{{\text{O}}_{3}}<\text{ N}{{\text{a}}_{2}}\text{C}{{\text{O}}_{3}}$.

Thus, the correct option is the option B.

Note: Do not consider hydrolysis of salt of strong acid with strong base. Such salts do not undergo hydrolysis.Such salt solutions have equal number of hydronium ions and hydroxide ions. Such salt solutions have pH equal to 7 and are neutral.