Question

True or False? Please, explain.

(a) Addition of a solute at constant T and P to a solution containing

solvent A always decreases μA.

(b) Addition of a nonvolatile solute at constant T to a pure solvent

always lowers the vapor pressure.

(c) For a binary two-phase system, the closer the point on a tie line is to a phase, the more of that phase is present.

(d) A system at equilibrium with substances i and k present in phase β must have μβi = μβk .

(e) The standard state activity is 0.

(f) In a solution of m moles of HC2H3O2 in 1 kg of water, the H3O+

molality can never exceed m mol/kg.

(g) The degree of dissociation of HC2H3O2(aq) is unchanged if some

NaCl is added.

(h) A weak acid is completely dissociated in the limit of infinite dilu-

tion in aqueous solution.

(i) The standard state is always chosen as a pure substance.

(j) ∆G◦ always refers to a transition from pure reactants to pure products.

Answer 1

a)

µ_A = µ_A⁰ + RT ln (x γ)

x = mole fraction of A

γ = activity coefficient of A

Therefore, addition of solute causes x to decrease and hence µ_A also decreases.

Statement is true.

b)

Lowering of vapor pressure is observed when non-volatile solute is added to pure solvent.

Statement is true.

c)

The closer the point on tie-line to a phase, more of that phase is present.

Statement is true.

d)

At equilibrium, chemical potential of all substances in a phase must be equal.

Statement is true.

e)

Activity of a substance in its standard state is 0.

Statement is true.

f)

CH₃COOH + H₂O --> CH₃COO^- + H₃O⁺

(m – x) --> x + x

Moles of H₃O⁺ can never exceed m moles/kg-water

x <= m

Statement is true.

g)

CH₃COOH + H₂O --> CH₃COO^- + H₃O⁺

Ka = [CH₃COO^-] [H₃O⁺] / [CH₃COOH]

NaCl will not affect the equilibrium attained among species present in the solution.

Cl^- is a very weak base.

Statement is true.

h)

Let initial of moles weak acid present be C and volume = V

HA + H₂O --> A^- + H₃O⁺

(C – x) --> x + x

Ka = [CH₃COO^-] [H₃O⁺] / [CH₃COOH]

= x²/ (V(C – x))

At infinite dilution, V tends to infinity. So (C – x) must tend to 0 for a finite Ka.

Hence, the weak acid completely dissociates. Statement is true.

i)

The standard state is always chosen as a pure substance. Statement is true.

j)

∆G◦, the standard free energy change represents the free energy change associated with the formation of the pure products from pure reactants (the elements in their most stable forms) as they exist under standard conditions.

It always refers to a transition from pure reactants to pure products.

Statement is true.