Question

A) What is the pH at the equivalence point in the titration of a 15.6 mL sample of a 0.312 M aqueous hypochlorous acid solution with a 0.302 M aqueous barium hydroxide solution?

pH = ___

B) When a 18.2 mL sample of a 0.426 M aqueous hypochlorous acid solution is titrated with a 0.458 M aqueous barium hydroxide solution, what is the pH at the midpoint in the titration?

pH = ___

Answer 1

A)

The reaction between Ba(OH)2 and HClO is

Ba(OH)2 + 2HClO -----> Ba(OCl)2 + 2H2O

1:2 reaction

No of moles of HClO = ( 0.312mol/1000ml)×15.6ml = 0.0048672 mol

No of moles of Ba(OH)2 = 0.0048672mol /2 = 0.0024336mol Volume of Ba(OH)2 required = (1000ml/0.302mol)×0.0024336mol = 8.06ml

Equivalence point = 8.06ml

Total volume at equivalence point = 8.06ml + 15.6ml = 23.66ml

At equivalence point,

[ClO^-] = (0.0048762mol/23.66ml)× 1000ml = 0.2057M

ClO^- is partly hydrolysed by water

ClO^- (aq) + H2O(l) <------> HClO(aq) + OH^-(aq)

K_b= [HClO][OH^-] /[ClO^-]

Ka of HClO = 2.95×10^-8

K_b= K_w/Ka

= 1.00×10^-14/2.95×10^-8

= 3.39× 10^-7

at equillibrium

[ClO^-] = 0.2057-x

[HClO] = x

[OH^-] = x

x²/(0.2058 -x) = 3.39×10^-7

we can assume 0.2058 -x ~ 0.2058 because x is small value

x²/0.2058 = 3.39×10^-7

x² = 6.98×10^-8

x = 2.64×10^-4

[OH^-] = 2.64×10^-4M

pOH = -log(2.64×10^-4)

pOH = 3.58

pH = 14 -3.58

pH = 10.42

B) For a weak acid titration, at equivalence point pH = pKa

pKa of HClO = 7.53

Therefore

At the mid point of titration , pH = 7.53