Question

1. Describe the effect on the equilibirum and the pH that results from the following additions.

a. addition of sodium dihydrogen phosphate to an aqueous solution of phosphoric acid.

b. addition of sodium hydroxide to an aqueous solution of methylammonium nitrate.

Answer 1

a. Addition of Sodium dihydrogenphosphate to aqueous phosphoric acid will result into,

(i) Shifting the equilibrium to LHS ( reactant side )

(ii) increase in pH value

Explanation -

This is a case of Phosphate buffer , where pH is given by the Handerson- Hasselbalch's expression.

pH = pKa + log { [ salt ] / [Acid ] }

Phosphoric acid H3PO4 ,ionizes in aqueous solution as,

H3PO4(aq) <------------ > H2PO4^- (aq) + H⁺ (aq)................................(i)

Now, when Sodium dihydrogenphosphate is added to it, more H2PO4^- ions are produced

Na H2PO4 (aq) ---------> H2PO4^- (aq) + Na⁺ (aq).............................................(ii)

This increases the concentration of the anion H2PO4^- in solution thereby pushing the equilibria (i)

to LHS yielding greater concentration of undissociated H3PO4 ( Common -ion -effect / as per

LeChatelier's principle )

Further, it amounts into decrease in [H⁺ ] and simultaneous increase in [OH^- ] in aqueos solution.

As a result pH of the solution should increase ( since , pH + P(OH) = 14 ). However the solution

under consideration is a buffer solution , so the extent of variation in pH would depend upon its

buffer- capacity also.

b.)

The addition of sodium hydroxide to a solution of methyl ammonium - nitrate results into,

(i) Shifting equilibrium to RHS

(ii) a decrease in pH

NaOH being a strong base is completely ionized to cause an excess of [OH^-] in solution. The OH^- so

yielded causes methyl ammonium ion to dissociate further due to reaction,

H⁺   + OH^- --------------> H2O

this shifts the equilibrium

CH3NH3⁺ <-------------> CH3NH2 + H⁺   to RHS, yielding more [H⁺]

Again , a higher concentration of H⁺ should result into decrease in pH