Question

In an excess of NH3(aq), Cu2+ ion forms a deep blue complex ion, Cu(NH3)42+, which has a formation constant Kf=5.6×1011. Calculate the concentration of Cu2+ in a solution prepared by adding 4.5×10−3mol of CuSO4 to 0.550 L of 0.40 M NH3. Express your answer using two significant figures.

Answer 1

Sol :-

Molarity = Number of moles / Volume of solution in L

So,

Molarity of Cu²⁺ = 4.5 x 10^-3 mol / 0.550 L = 0.00818 M

ICE table of the reaction is :

..............................Cu²⁺ .............+....................4 NH₃ ---------------------------------> [Cu(NH₃)₄]²⁺

Initial (I)..................0.00818 M..............................0.40 M...........................................0.0 M

Change (C).............-y..............................................-4y.................................................+y

Equilibrium (E).......(0.00818-y) M.........................(0.40-4y) M.......................................y M

y = Amount dissociated per mole.

Expression of formation constant (K_f) is :

K_f = [Cu(NH₃)₄]²⁺ / [Cu²⁺].[NH₃]⁴

5.6 x 10¹¹ = y/(0.00818-y).(0.40-4y)⁴

5.6 x 10¹¹ x (0.00818-y).(0.36)⁴ = y

On solving

y = 1.0 x 10^-2 M

Therefore, [Cu²⁺] = 1.0 x 10^-2 M