Q. 10.58 - An Introduction to General, Organic, and Biological Chemistry

Question

Consider the buffer system of nitrous acid, ${\mathrm{HNO}}_2$, and its salt, ${\mathrm{NaNO}}_2$.

${\mathrm{HNO}}_2\left(aq\right)+{\mathrm{H}}_2\mathrm{O}\left(l\right)\rightleftarrows {\mathrm{H}}_3{\mathrm{O}}^{+}\left(aq\right)+{\mathrm{NO}}_2^{-}\left(aq\right)$

a.   The purpose of this buffer system is to:

             1. maintain $\left[{\mathrm{HNO}}_2\right]$

             2. maintain $\left[{{\mathrm{NO}}_2}^{-}\right]$

             3. maintain $\mathrm{pH}$

b.  The weak acid is needed to:

             1. provide the conjugate base

             2. neutralize added ${\mathrm{OH}}^{-}$

             3. provide the conjugate acid

c.  If ${\mathrm{H}}_3{\mathrm{O}}^{+}$is added, it is neutralized by:

             1. the salt

             2. ${\mathrm{H}}_2\mathrm{O}$

             3. ${\mathrm{OH}}^{-}$

d. When ${\mathrm{OH}}^{-}$is added, the equilibrium shifts in the direction of the:

             1. reactants

             2. products

             3. does not change

Step-by-Step Solution

Answer

a)  The purpose of this buffer system is to maintain $\mathrm{pH}$.

b)  The weak acid is needed to neutralize ${\mathrm{OH}}^{-}$.

c)  If  ${\mathrm{H}}_3{\mathrm{O}}^{+}$ is added, it is neutralized by ${\mathrm{OH}}^{-}$.

d)  When ${\mathrm{OH}}^{-}$ is added, the equilibrium shift in the direction of reactants.

Step - 1 Introduction

Buffer solution:

                         A weak acid and its conjugate base react with a strong base to form a buffer solution. When modest amounts of acid or base are introduced to this solution, the $\mathrm{pH}$ remains unaltered.

Step - 2 Given Information (part-a)

a)   Nitrous acid and its salt have the following buffer system:

    ${\mathrm{HNO}}_2\left(aq\right)+{\mathrm{H}}_2\mathrm{O}\left(l\right)\rightleftharpoons {\mathrm{H}}_3{\mathrm{O}}^{+}\left(aq\right)+{\mathrm{NO}}_2^{-}\left(aq\right)$

   

Step - 3 Explanation (part-a)

• A buffer solution's pH is generally constant. 
• The goal of this buffer system is to keep the $\mathrm{pH}$of this solution constant. .

Step - 4 Given information (part-b)

      The weak acid is required for:

Step - 5 Explanation (part- b)

• The conjugate base and acid are not provided by the weak acid in this buffer system. 
• It is essential to neutralize the additional ${\mathrm{OH}}^{-}$.

Step - 6 Given information (part-c)

 

 If you add  ${\mathrm{H}}_3{\mathrm{O}}^{+}$ to the equation, it is neutralized by:

Step -7 Explanation (part-c)

When acid or ${\mathrm{H}}_3{\mathrm{O}}^{+}$ is supplied to the buffer system, it is neutralized by the ${\mathrm{OH}}^{-}$reaction, which produces water.

Step -8 Explanation (part-d)

• The equilibrium shifts in the direction of consumption when base or ${\mathrm{OH}}^{-}$ is added to the buffer system.
•  When ${\mathrm{H}}_3{\mathrm{O}}^{+}$reacts, base is consumed.
•  As a result, the reaction proceeds in the same direction as the reactants.