3. Consider the following situation involving aqueous ammonia and Cu? a. List al

Question

3. Consider the following situation involving aqueous ammonia and Cu? a. List all species present in a 1.0 M ammonia solution. b. Is the pH of the ammonia solution acidic or basic? c. Looking at both the Ksp and Kf data given in Section 4.2, what reactions might occur when a 1.0 M ammonia is added to a Cu+ solution? has happened? What is the most likely product? d. does not dissolve the precipitate but addition of 15.0 M ammonia does. d. If a solid precipitate is formed when 1.0 M ammonia is added to Cu*, what e. Further addition of 1.0 M ammonia to the solution and precipitate from part Explain. Lab 10 (Metal Cations)-Page 13 of 20

Explanation / Answer

a. Since ammonia is a weak base:

NH3(aq) + H2O(l) -----> NH4+(aq) + OH-(aq); pKb = 9.25

b.

Since ammonia is a weak base, the concentration of hydroxyl ions is given by,

[OH-] = sqrt(Kb * [NH3])

Kb is the ionisation constant of ammonia. It's value is approximately equal to 2× 10^-5.

[OH-] = sqrt(2× 10^-5 * 1) = 0.0044 M

We know that for aqueous solutions, at 298K

[H+][OH?]=10?¹?

[H+][0.0044]=10?¹?

[H+] = 2.27 * 10^-12

From this, we can calculate [H+]

pH=?log[H+] = -log(2.27 * 10^-12) = 11.64

which proves ammonia is a weak base.

c. d. Copper(II) ion reacts with stoichiometric quantities of aqueous ammonia to precipitate light blue Cu(OH)2

Cu2+(aq) + 2NH3(aq) + 3H2O(l) -----> Cu(OH)2(s) + 2NH4+(aq)?

e. The precipitate Cu(OH)2 dissolves in excess ammonia to form a dark blue complex ion:

Cu(OH)2(s) + 4NH3(aq) <==> [Cu(NH3)4]2+(aq) + 2OH-(aq)

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