Question

For each of the following, is the solution acidic, basic, neutral, or cannot be determined? For each, write the equation for the dominant equilibrium which determined the pH, and justify your pH prediction. Kw = 1.0 x 10-14

100 mL of 0.10 M NaH3P2O7; Ka1 = 3.0 x 10-2, Ka2 = 4.4 x 10-3, Ka3 = 2.5 x 10-7, and Ka4 = 5.6 x 10-10 for H4P2O7.

Answer 1

Write down the step-wise dissociation of H₄P₂O₇ as below:

(1) H₄P₂O₇ (aq) <=====> H⁺ (aq) + H₃P₂O₇^- (aq)    K_a1 = 3.0*10^-2

(2) H₃P₂O₇^- (aq) <====> H⁺ (aq) + H₂P₂O₇^2- (aq)    K_a2 = 4.4*10^-3

(3) H₂P₂O₇^2- (aq) <====> H⁺ (aq) + HP₂O₇^3- (aq)    K_a3 = 2.5*10^-7

(4) HP₂O₇^3- (aq) <=====> H⁺ (aq) + P₂O₇^4- (aq)      K_a4 = 5.6*10^-10

NaH₃P₂O₇ dissociates as Na⁺ and H₃P₂O₇^- and equilibrium (2) will be operational. H₃P₂O₇^2- is amphoteric and hence the pH will be given by pH = ½*(pK_a1 + pK_a2)

Given K_a1 = 3.0*10^-2, pK_a1 = -log (K_a1) = 1.523; K_a2 = 4.4*10^-3, pK_a2 = -log (K_a2) = 2.356; therefore,

pH = ½*(1.523 + 2.356) = 1.9395 ≈ 1.940

Since the pH of the solution is low, therefore, the solution will be acidic (ans).