Question

# For each of the following, is the solution acidic, basic, neutral, or cannot be determined? For...

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.

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

(1) H4P2O7 (aq) <=====> H+ (aq) + H3P2O7- (aq)    Ka1 = 3.0*10-2

(2) H3P2O7- (aq) <====> H+ (aq) + H2P2O72- (aq)    Ka2 = 4.4*10-3

(3) H2P2O72- (aq) <====> H+ (aq) + HP2O73- (aq)    Ka3 = 2.5*10-7

(4) HP2O73- (aq) <=====> H+ (aq) + P2O74- (aq)      Ka4 = 5.6*10-10

NaH3P2O7 dissociates as Na+ and H3P2O7- and equilibrium (2) will be operational. H3P2O72- is amphoteric and hence the pH will be given by pH = ½*(pKa1 + pKa2)

Given Ka1 = 3.0*10-2, pKa1 = -log (Ka1) = 1.523; Ka2 = 4.4*10-3, pKa2 = -log (Ka2) = 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).