[bokwandu]

A solution is made that is 1.1×10−3 M in Zn(NO_3)_2 and 0.150 M in NH_3.

After the solution reaches equilibrium, what concentration of Zn^2+ (aq) remains?

[bokwandu]

alright so far i have Kf = 4.1 x 10^8 for the formation of [Zn(NH_3)_4]^2+ from Zn^2+ and 4NH_3.
And Kf = [Zn(NH_3)_4] / [Zn^2+]x[NH_3]^4.
I set up an ice table to find the change in Zn^2+ but the numbers aren't coming out right... is there something im doing wrong or missing?

[bokwandu]

i keep getting 0 but its wrong... XD

[Borek]

Show how you got it.

[bokwandu]

I set up the ice table to look like
          Zn             +           4NH_3       =       (Zn[NH_3]_4)
I     1.1*10^-3 M               .150 M                    0 M
C        -X                            -4X                      +X
E   1.1*10^-3 - X               .150 - 4X                X


after substituting into the Ksp equation, i get X/(1.1*10^-3-X)(.150-4X)^4=4.1*10^8
I assume that X is negligible, or really small compared to .150 so i simplify (.150-4X)^4 into (.150)^4.
After solving it out i get X = 1.1*10-3 and 1.1*10^-3 - 1.1*10^-3 = 0

[Borek]

It's not that wrong

It means that almost all zinc has been complexed. If so, you may safely assume concentration of complex to be equal to initial concentration of zinc.

[bokwandu]

Hmmm that would mean there is basically 0 g of Zn2+ left in the solution, but when i entered that number it came out as incorrect. I don't know what to do next... maybe the professor entered the answer wrong?

[Borek]

No, solve Kf for free Zn²⁺ concentration and put ammonia as 0.15 and complex as 1.1x10^-3. Both values are close to reality.

[AWK]

No, solve Kf for free Zn²⁺ concentration and put ammonia as 0.15 and complex as 1.1x10^-3. Both values are close to reality.

Slightly better as 0.15 - 4x0.0011 = 0.1456

[bokwandu]

ugh, i still got the wrong answer in the end, they have a recorded answer of 1.2*10^-9... and i havent the slightest clue as to how they got that answer. thanks anyways though guys

[Borek]

Their answer is wrong.

First attachement: equilibrium calculated ignoring fact that NH₃ is a base.

Second attachement: NH₃ reaction with water taken into account. I thought that's what makes the difference, but that's not the case.

[AWK]

Borek,
what about Ksp of Zn(OH)2

[Borek]

Could be that's what they aim at. A lot depends on the pKso value.

What I at hand is 15.7, which gives 2 x 10^-10 (if ammonia complex is ignored) or 2.5 x 10^-10 if it is not.

Other version - they want bokwandu to precipitate Zn(OH)₂ and ignore fact that it changes pOH. 0.15M ammonia gives pH 11.21, this leads to [Zn²⁺] = 7.6 x 10^-11.

None fits.