Q.(a)

This choice question asks either for definitions of isotonic solution and the ideal gas equation, or for the derivation of the Kp–Kc relation. …

This is a 5-mark either/or question; both alternatives are worked out below.

PART (a)(i) — Isotonic solution:

Two solutions are said to be isotonic if they exert the same osmotic pressure at the same temperature. When such solutions are separated by a semipermeable membrane, there is no net flow of solvent from one to the other (they have equal solute-particle concentration). Solutions of higher osmotic pressure are hypertonic, and those of lower osmotic pressure are hypotonic, relative to one another.

PART (a)(ii) — Ideal gas equation:

The ideal gas equation is PV = nRT, where P = pressure, V = volume, n = number of moles of the gas, R = universal gas constant (8.314 J mol^-1 K^-1) and T = absolute temperature. It is obtained by combining Boyle's law (V ∝ 1/P), Charles's law (V ∝ T) and Avogadro's law (V ∝ n), giving V ∝ nT/P, i.e. PV = nRT. It describes the state of an ideal gas.

PART (b) — Relation between Kp and Kc:

Consider a general gaseous equilibrium: aA + bB ⇌ cC + dD.

Kp is expressed in terms of partial pressures and Kc in terms of molar concentrations:

Kp = (pC^c · pD^d) / (pA^a · pB^b); Kc = ([C]^c [D]^d) / ([A]^a [B]^b). …