Question

(c)     The data set below represents the surface tension data for aqueous solutions of 3-methylbutanoic acid [(CH₃)₂CHCH₂COOH] at 25^oC.

[C] [mol.dm^-3]    0          0.009               0.026               0.065                     0.135

γ [mN.m^-1]          72.7     64.6                 59.0                                51.3                       47.0

(i) Calculate the surface excess of 3-methylbutanoic acid (in mol.m^-2) and

(ii) Hence estimate the molecular cross sectional area (in nm²) for this surface active molecule.

(iii)   Compare your estimate with the molecular cross sectional area one might expect for n-butanoic acid and account for the difference, if any, between the two surface active molecules.

Answer 1

Refer image for (i) and (ii)
(iii) Generally surface tension decreases with the increase in molecular mass.Since molecular mass of 3-methylbutanoic acid is more than that of n-butanoic acid.Hence,Surface Tension of 3-methylbutanoic acid is less.Since molecular cross sectional area is inversely proportional to Surface Tension.Therefore, molecular cross sectional area of 3-methylbutanoic acid will be greator than that of n-butanoic acid.