If 50 ml of .10 M zinc bromide is mixed with
50 ml of 2 M NaOH.
What is the concentration of Zn2+?
So before we start this reaction, in fact
I will set it up so you understand what you
need to be doing here.
We’re going to set up zinc, it’s going
to react with four hydroxides to form Zn(OH)4
2- with four hydroxide ligands.
What we need to do is figure out the concentration
of zinc initially and the concentration of
hydroxide initially.
So here’s my 50 ml times the concentration,
divided by 100 ml the total.
That’s going to give me .050 M Zn2+.
For hydroxide, I have .050 L times the concentration
of 2 M divided by the total volume, which
is 100 ml or .1 L. The zinc’s initial concentration
is .050 M. While the hydroxides concentration
is .95 M and the complex ion is zero.
We’re just setting this up to make the math
easier.
These are all before the reaction begins.
Now the change, this is gonna go down and
assume its gonna be all used up, and what
goes here is a -4(.050) and this complex ion
is going to go up .050.
This is going to be zero, this is going to
be .80 and this is going to be .050.
Those are going to be our initial concentrations.
Now we’re going to set up the equilibrium.
I’m going to set up the same equation and
bring those same values down that we found
in the previous step.
This will be our ICE box.
Now our change will be plus x, this will go
up 4x, meaning .80 + 4x, and lastly, this
will go down x.
Meaning the equation will be .050-x.
Because our mol ratio is 1/1.
The Kf value, looking that up is a huge K
value at 3.0x10^15.
Which means the equilibrium favors the products
very heavily.
Okay, so Kf is going to equal the conetration
the complex ion over 
the zn2+ and the OH- ^4.
Lets plus in our numbers, now this is going
to 
be .05- x over x, (.80+x)^4.
Now can we neglect?
Yes, we’re going to have to in order to
be able to not have to deal with the power
of four.
Okay, so our equation becomes 3.0 x10^15=
.0500/(x)(.80)^4, so x=.0500/(3.0x10^15)(.80)^4.
X turns out to be 4.1 x10^-17, that’s going
to equal my concentration of Zn2+.
Now let’s make sure our neglecting x was
okay.
So, 4.1x10^-17 /.050 x 100= that’s going
to be way less than 5%.
So yes we can neglect x.
