The equilibrium constants (ka) for hcn and hf in h2o at 25 degrees are 6.2e-10 and 7.2e-4,respectively. the relative order of base strengths including water is:

1) HCN

2) HF

Explanation:

∴ Ka1 = [H+]*[CN-] / [HCN] = 6.2 E-10

⇒ pKa1 = - log Ka1 = 9.2076

∴ Ka2 = [H+]*[F-] / [HF] = 7.2 E-4

⇒ pKa2 = - Log Ka2 = 3.1426

∴ Kw = [H3O+]*[OH-] = 1 E-14

∴ pKw = 14

∴ pKw = pKa + pKb

⇒ pKb1 = 14 - pKa1 = 14 - 9.2076 = 4.792

⇒ pKb2 = 14 - pKa2 = 14 - 3.1426 = 10.857

∴ pKb2 > pKb1

⇒ the relative order of base strengths:

1) HCN

2) HF

Because a reducide pKb is associated with a stronge base, while a large pKb revels a weak base.

vanderwaal's forces of attraction are the electrostatic intermolecular forces that exist in a molecule due to the temporary or permanent dipoles that are developed in a molecule. in , the major intermolecular forces are dipole-dipole interactions as the molecule is polar (bent structure). so the vanderwaal's forces of interaction in between molecules are the permanent dipole-dipole interactions that develop due to the dipole that exists in between the polar h - s bond. the positively charged hydrogen atom is attracted towards the negative end of the dipole that is sulfur of the adjacent molecule. therefore, these forces of attraction are relatively stronger than that of london dispersion forces that exist in non polar molecules.