Hydrogen peroxide, H₂O₂, is a somewhat stronger acid than water. Its ionization is represented by the equation

In 1912, the following experiments were performed to obtain an approximate value of pK_a for this ionization at 0 °C. A sample of H₂O₂ was shaken together with a mixture of water and pentan-1-ol. The mixture settled into two layers. At equilibrium, the hydrogen peroxide had distributed itself between the two layers such that the water layer contained 6.78 times as much H₂O₂ as the pentan-1-ol layer. In a second experiment, a sample of H₂O₂ was shaken together with 0.250 M NaOH(aq) and pentan-1-ol. At equilibrium, the concentration of H₂O₂ was 0.00357 M in the pentan-1-ol layer and 0.259 M in the aqueous layer. In a third experiment, a sample of H₂O₂ was brought to equilibrium with a mixture of pentan-1-ol and 0.125 M NaOH(aq); the concentrations of the hydrogen peroxide were 0.00198 M in the pentan-1-ol and 0.123 M in the aqueous layer. For water at 0 °C, pK_w = 14.94. Find an approximate value of pK_a for H₂O₂ at 0 °C. The hydrogen peroxide concentration in the aqueous layers is the total concentration of H₂O₂ and HO₂^-. Assume that the pentan-1-ol solutions contain no ionic species.

Transcribed Image Text:

H₂O2 + H₂O H3O+ + HO₂