The degree of dissociation alpha of propanic acid, CH3CH2CO2H, in aqueous soluti

Question

The degree of dissociation alpha of propanic acid, CH3CH2CO2H, in aqueous solution at 298K varies with concentration c in the following manner:

c/mol dm^-3...............2.801x10^-5...........1.532x10^-4..............2.414x10^-3..............5.912x10^-3

alpha............................0.5393....................0.2875........................0.0829.......................0.054

a) The degree of dissociation is the extent of reaction of the acid-dissociation reaction. relate the ionic strength of the solution to alpha and c.

b) Derive an expression that relates the thermodynamic acid dissociation constant, Ka, to alpha, c, and gamma+/- (activity coefficient).

c) show how you can manipulate your expression from (b) to fit the data in the table (above) to a straight line using Debye-Huckel theory: (loggamma_{+/-} = -0.509[z^{+}z^{-}]sqrt{I})

d) carry out the analysis from (c) and calculate the dissociation constant of propanoic acid. How far do the data verify the Debye-Huckel theory?

Explanation / Answer

a)CH3CH2COOH------------------> CH3CH2COO- +H+

K = alpha^2/(c-alpha)

where K is dissociation constant

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