Question

What is the pH of a 0.00100 F solution of hypobromous acid (HOBr) in 0.0500F NaCl? (αOBr- ≈ 350 pm)

(Hint: Remember, neutral species do not have activity coefficients (they are not affected by ionic strength))

Answer 1

u = 1/2(Ci.zi^2)

Ci = concentration of ion

zi= charge on ion

for 0.05 M NaCl

u = 1/2(0.05 * 1^2 + 0.05 * 1^2)

Then Calculate

u= 0.05 M

Debye-Huckel equation for activity coefficient [Y]

log[Y] = -0.51 * zi^2 * sq.rt.(u)/[1 + 3.3 * alpha * sq.rt.(u)]

with alpha being diameter of ion

log[Y] for H3O+ = -0.51 * 1^2 * sq.rt(0.05)/[1 + 3.3 * 0.9 * sq.rt.(0.05)

or activity coefficient [Y] for H3O+ = 0.854

HOBr + H2O <==> H3O+ + OBr-

Ka = [H3O+][OBr-]/[HOBr]

2.3 * 10^-9 = x^2/0.001

x = [H3O+]

x= 1.516 x 10^-6 M

pH = -log([Y]forH3O+ * 1.516 * 10^-6)

pH= 5.89

pH of 0.001 M HOBr solution is 5.89