Question

7)pKa for phenophthalein is 9.3 at 25C

a) Calculate ratio of its anionic form to acid form at pH 8.2 and at pH 10.

b) Using these values, explain the color change within this pH range.

8)

Given the following data, calculate the ΔG (KJ) for this reaction and state whether it is spontaneous.
CH3CH2OH + O2 -> CH3COOH + H2O
   ΔH = -0.495200 MJ
   ΔS = -136 J/K
   T = 25C

Answer 1

7)   for buffer system

pH= pka + log(salt/acid)

a) pka = 9.3

at pH = 8.2

8.2 = 9.3 + logx

x = (salt/acid) = (anionic/acid) = 0.08

at pH = 10

10 = 9.3 + logx

x = (salt/acid) = (anionic/acid) = 5

b) from the above information the ratio changes from 0.08 to 5. means acid converts in to anionic state.so that color

changes from colorless to pink.

pHenopthalein neotral = colorless

               anionic form = pink

8)

   CH3CH2OH + O2 -> CH3COOH + H2O
   ΔH = -0.495200 MJ
   ΔS = -136 J/K
   T = 25C

DGrxn = DHrxn - TDSrxn

         = (-0.495*10^6) - (-136*298)

         = - 454.472 kj

as DG = - ve it is spontaneous.