Question

An aqueous solution is 0.10 M in Na₂SO₄ and also 0.15 M in KBr.

(a) What is the ionic strength of the mixture?

(b) Which salt makes a greater contribution to the ionic strength? Why?

(c) What impact does the ionic strength have on the activity coefficients and activity of each ion? (i) both activity coefficient and activity increase

            (ii) both activity coefficient and activity decrease

            (iii) activity coefficient decreases and activity increases

            (iv) activity decreases but activity coefficient increases

(d) Write the charge balance equation for this mixture.

(e) Write the mass balance equation for the acid system involving the SO₄^{2 ̶} ion. Assume no ion pairing occurs.

Answer 1

For 0.10 M Na₂SO₄ the ionic strength is

=1/2{[Na+]×(+1)²+[SO₄^2-]×(−2)²}

=1/2{(0.10)×(+1)²+(0.20)×(−2)²}

        =0.30 M

The ionic strength for 0.15 M KBr is

=1/2{[K+]×(+1)²+[Br^-]×(−1)²}

=1/2{(0.15)×(+1)²+(0.15)×(−1)²}

=0.15 M

total 0.10 x 0.15 = 0.015 M

2) that the unit for ionic strength is molarity, but that a salt’s ionic strength need not match its molar concentration. For a 1:1 salt, such as KBr, ionic strength and molar concentration are identical. The ionic strength of a 2:1 electrolyte, such as Na₂SO₄, is 1.5 times larger than the electrolyte’s molar concentration.

3) ΔG∘=−RTlnK

Because a reaction has only one standard-state, its equilibrium constant must be independent of solution conditions. Although ionic strength affects the apparent formation constant for Na2SO4, reaction must have an underlying thermodynamic formation constant that is independent of ionic strength.

The apparent formation constant for Na2SO4, is a function of concentrations. In place of concentrations, we define the true thermodynamic equilibrium constant using activities. The activity of species A, a_A, is the product of its concentration, [A], and a solution-dependent activity coefficient,γ_A.

aA=[A]γA

The true thermodynamic formation constant for Na2SO4, therefore, is

K1= a Na2SO4 / aNa⁺aSO₄^2-=   [Na2SO4] γNa2SO4 / [Na⁺] γNa⁺[SO₄2-] γSO₄^2-

option A is correct

d) Na2SO4 ---> 2Na⁺ +    SO4^2-

KBr -----> K ^- + Br ^-

e) one mol of Na2SO4 produce 2 moles of Na⁺ and one mol of SO4^2-   

and in KBr one mol produce one mol of K⁺ and one mol of Br^-