What is Kc for the following equilibrium? For phosphoric acid, Ka1=6.9x10^-3, Ka2=6.2x10^-8 and Ka3=4.8x10^-13
HPO4^-2 + OH^- <==> PO4^-3 + H2O
Sara, this is a reverse hydrolysis problem. See your problem above for how that works.
So Kb for PO4^3- = (Kw/k3 for H3PO4) = ?
Then for the reverse hydrolysis it will be Kc = 1/Kb.
I plug in the numbers put it doesn't match any of the choices.... I get 4.81x10^-29
The choices are
A)2.3x10^-35
B)6.4x10^-23
C)7.3x10^-16
D)48
E)491
Please help!!
I don't know which buttons you punched but when I crunched the numbers I got 48 and that is one of the answers listed. I'd be interested in knowing what you did? Did I say something that was difficult to understand?
Kb phosphate = (kw/k3) = 1E-14/4.8E-13 = 0.02083 = Kb for phosphate.
1/Kb = 1/0.0203 = 48.0
I don't press the E button i press the 10^x
Button but when I divided the kw/Kb it gives me 2.083x10^-28
Take a look at your calculator instruction manual for dividing one number by the other.
(Kw/k3) = 1 x 10^-14/4.8 x 10^-13 = 0.02038
1/0.02038 = 48.0
To find Kc for the given equilibrium, we need to write the balanced equation first. The equation is:
HPO4^2- + OH^- ⇌ PO4^3- + H2O
Next, we need to write the expression for the equilibrium constant Kc using the concentrations of the species involved. The general form of the expression is:
Kc = [C]^c [D]^d / [A]^a [B]^b
Here, [A], [B], [C], and [D] represent the concentrations of the species on each side of the equation, and a, b, c, and d represent the stoichiometric coefficients of the species.
In the given equilibrium, the stoichiometric coefficients are:
HPO4^2- + OH^- ⇌ PO4^3- + H2O
1 + 1 ⇌ 1 + 1
Therefore, the expression for Kc is:
Kc = [PO4^3-] [H2O] / [HPO4^2-] [OH^-]
Now, we need to find the concentrations of the species at equilibrium. To do that, we need to look at the acid dissociation constants (Ka) for the relevant species.
Ka1 = [H+][HPO4^2-] / [H3PO4]
Ka2 = [H+][PO4^3-] / [HPO4^2-]
Ka3 = [H+][H2O] / [PO4^3-]
Since we have the Ka values, we can use these equations to find the concentrations of the species at equilibrium.
Given that:
Ka1 = 6.9x10^-3
Ka2 = 6.2x10^-8
Ka3 = 4.8x10^-13
We can use these equations to solve for the concentrations at equilibrium. Once we have the concentrations, we can substitute them into the Kc expression to find the value of Kc.