Question

yellow blue or HIn is a weak acid with a pK, value of 5.0. It reacts with water as represented in the sider the two beakers below. Each beaker has a layer of colorless oil (a nonpolon aqueous buffer layer of buffer solution is 7. A small and water layers are allowed to separate ucous buffer solution. In beaker X the pH of the buffer solution is 3, and in beaker Yth wate A small amount of HIn is placed in both beakers. The mixtures are stired well, and t te solvent) are stirred well, and the ol Oil Oil Buffer pH 3 Beaker X Buffer PH 7 Beaker Y (a) What is the predominant form of HIn in the aqueous buffer in beaker Y, the acid form or the conjugate base form? Explain your reasoning. (b) In beaker X the oil layer is yellow, whercas in beaker Y the oil layer is colorless. Explain these observations in terms of both acid-base equilibria and interparticle forces.

Answer 1

The pKa of the indicatior HIn is the negative logarithm of the equilibrium constant of the indicator in its protonated and deprotonated states. Thus at a pH equal to the pKa of HIn, the concentrations of HIn and In^- will be exactly equal. Being a weak acid, at acidic pH that is till pH = 5 for HIn, the indicator stays protonated and the predominant form is yellow and in basic media the indicator starts deprotonating rapidly and the predominant form becomes the blue In^-.

a) From the above explanation it can now be reasoned that the predominant form of the indicator in beaker Y is the blue form as the indicator starts to ionize rapidly at pH above 5, its pKa and this blue form corresponds to its conjugate base.

b) In beaker X due to the acidic nature of the aqueous layer, the indicator stays unionized and is hence bereft of any charge-separation. Water being a polar solvent interacts only with ions while oil is non-polar and interacts using van der Waals forces and such hydrophobic interactions with its solutes. Here since the indicator is not as ions but bound as a non-ionic entity, it is more soluble in the non-polar oil thus giving its unionized form's yellow colour in the beaker X. In beaker Y however, the indicator is ionized largely and thus exists as ions which dissolve very well in water and not in oil hence the lack of colour in oil while imparting a blue colour to the aqueous layer.