Chem

posted by .

What is the equilibrium constant expression for the predominant equilibrium in HPO4^2- (aq)?

• Chem -

HPO4^-2 ==> H^+ + PO4^-3 and

The first one is
K3 = (H^+)(PO4^-3)/(HPO4^-2)

• Chem -

I answered that, but the teacher marked me wrong.. so I don't understand why?
The predominant reaction should be HPO4^2- acting as an acid right?

• Chem -

I wrote, then erased before I posted the response that there could be two competing reactions. The second one was
H^+ + H2PO4^-2 ==> H2PO4^-
Written as it is, this is the reverse of k2 and with a Keq of about 100,000 or so it could be. The problem with problems like this is one never knows exactly what predominant means (except to the person making up the problem).

• Chem -

I don't really understand what k2 and k3 means..
To find the predominant reaction, could I compare the ka and kb values for HPO4^2-?

• Chem -

k1 = (H^+)(HPO4^-2)/(H3PO4 = about 1E-2
k2 = (H^+)(HPO4^-2)/(H2PO4^-) = about 1E-8
k3 = (H^+)(PO4^-3)/(HPO4^-2) = about 1E-13

Here is what k1, k2, and k3 mean.
And yes, I don't see why you can't compare Ka and Kb to determine the reaction most likely to occur.

• Chem -

So I found the Ka for HPO4^2- which is 2.2x10^-13
Then i found HPO42- on right side of the table and found the ka value which is 6.2x10^-8.
Then, I used kb=kw/ka to and got 1.61x10^-7
Since kb>ka the reaction:
HPO4^2- + H2O <--> H2PO4- + OH-
would be the one most likely to occur?

Similar Questions

1. chemistry

Determine the expression for the equilibrium constant for this equilibrium reaction: 2NO(g) + 2CO(g) (arrows)(in equilibrium with) N2(g) + 2CO2(g) PLease help anyone i really apreciate it Beccy
2. Chemistry

In the industrial synthesis of ammonia, the equilibrium constant expression may be written as: Keq= [NH3]^2/[N2][H2]^3 Calculate the value of this equilibrium constant, if the equilibrium concentration of nitrogen in the reaction mixture …
3. Chemistry

Which of the following statements are incorrect?
4. AP CHEMISTRY

write the equilibrium-constant expression and calculate the value of the equilibrium constant for each of the following reactions at 298 K. (a) NaHCO3(s)<--> NaOH(s) + CO2(g) Answer: Keq = PCO2 (b)Value of the Equilibrium Constant: …
5. Chemistry

SO this is my first time doing this... lol i need help on an AP chemistry question for equilibrium. A 0.500 L tank contains 3.00 g of NO(g) at 750. K. The equilibrium constant for the reaction below at this temperature is 3.4 x 10 …
6. Chemistry

Write a balanced equilibrium equation and kb expression for the reaction of the hydrogen phosphate ion (HPO4^2-) with water. Answer: HPO4^2-(aq) +H2O(l) <--> PO4^3-(aq) +H3O+ (aq) kb= [PO4^3-][H3O+]/ [HPO4^2-] is that right?
7. chemistry

Concerning the following reaction at equilibrium: 3Fe(s) + 4H2O(g) Fe3O4(s) + 4H2(g), increasing the concentration of the Fe(s) would: Answer A. Shift the equilibrium to the right B. Shift the equilibrium to the left C. No change D. …
8. CHEMISTRY

A chemist studying the equilibrium N2O4(g)<----->2NO2(g) controls the temperature so that keq ( equilibrium constant)= 0.028. At one equilibrium position, the concentration of N2O4 is 1.5 times greater than the concentration …
9. chem

What is constant about the equilibrium constant?
10. chem help!

If the equilibrium constant is much less than 1, how can you tell where the equilibrium lies?

More Similar Questions