In excess of NH3(aq), Zn2+ forms a complex ion, [Zn(NH3)4]2+ which has a formation constant Kf = 7.8 x 10^8. Calculate the concentration of Zn2+ in a solution prepared by adding 1.00 x 10^-2 mol Zn(NO3)2 to 1.00 L of 0.250 M NH3.
(a) 7.9 x 10^-4 M
(b) 2.8 x 10^-6 M
(c) 3.9 x 10^-9 M
(d) 6.4 x 10^-11 M

Question

In excess of NH3(aq), Zn2+ forms a complex ion, [Zn(NH3)4]2+ which has a formation constant Kf = 7.8 x 10^8. Calculate the concentration of Zn2+ in a solution prepared by adding 1.00 x 10^-2 mol Zn(NO3)2 to 1.00 L of 0.250 M NH3. 
(a) 7.9 x 10^-4 M
(b) 2.8 x 10^-6 M
(c) 3.9 x 10^-9 M
(d) 6.4 x 10^-11 M

Accepted Answer

All right. Hi, everyone. So this question says to deduce each central metals, co-ordination number of the following complex ions. Part one is P DC 42 negative and part two is co EDT A NH 323 negative. Here we have four different answer choices, proposing different co-ordination numbers or compounds, one and two or ions, one and two. So recall first and foremost that when it comes to the central metal in a complex ion, the co-ordination number refers to the number of li and coordinations sites or it refers to the total number of coordinate bonds formed between each ligand and the central atom or ion itself. Now, the co ordination number depends on what types of ligands are present in the compound itself because we can differentiate between mono dentate bate trident, et cetera. Now, mono dentate leggings, mono dentate ligands bind to the central metal ion via one donor. The donor add up, which means that there is only one attachment site between the mono dentate ligand and the central metal ion itself. Which means that mono dentate ligands tend to contribute one co-ordination number because they form one coordinate bond. Similarly, right, buy dentate leggings use two donor Adams to bind to the central metal ion and therefore bind to the central metal ion at two different points contributing a co-ordination number of two and so on. So let's start with the first one with part one, right? And that is P DC L 42 negative. Now, our central metal atom happens to be palladium and we only have one type of like, right? It just so happens that we only have or chloride ligands. Now recall that chlorine is mono dentate, which means that for every chlorine atom present in this complex, the co-ordination number increases by one. So chlorine contributes a co ordination number of one which when I multiply that by four gives me a total co ordination number of four or the first complex ion. So that's the co-ordination number of the first ion which is for. But now we can do the same thing for our second complex ion which is C eta NH 32, three negative. Now our central atom in this case is Cobalt and it just so happens that we have two different types of ligands. We have one molecule of EDTA as well as two molecules of ammonia. Now recall that EDT A is actually a hexad dentate ligand, which means that it can form six coordinate bonds with Cobalt. So EDTA contributes a coordinating number of six to this complex. And because I have only one molecule of EDTA six multiplied by one gives me six. Now by contrast, ammonia happens to be monogenic. So each molecule of ammonia that I have contributes a co ordination number of one. Now it just so happened that I have two molecules of ammonia. So I will multiply one by two to give me two. So when I add together my co-ordination numbers six from EDTA and two from my two molecules of ammonia, that gives me a total coordinating number of eight for my second complex ion. And there you have it, that means that our answer is going to be option D in the multiple choice because the co ordination number of the first complex ion is four and eight for the second one specifically for their metal centers. And with that being said, thank you so very much for watching. And I hope you found this helpful.

Verified Solution

Key Concepts

Complex Ion Formation

Formation Constant (Kf)

Equilibrium Calculations