What is the pH of a 0.1M Na2HAsO4 solution?

[Nikitin]

Homework Statement

The cid constants for the polyprotic acid H3AsO4 are Ka1=5x10^-3, Ka2=8x10^-
8, Ka3=6x10^-10. What is the pH of a 0.1M Na2HAsO4 solution?

Homework Equations

Kb = [H2A]*[OH]/[HA]
mass balance, charge balance.. etc.

The Attempt at a Solution

Well I've tried setting up a bunch of equations but I can't do this. Does anyone have any tips?

 

[Nikitin]

never mind.. I asked a classm8 and he told me about the pH=pka1+pka2 /2 equation.

 

[AGNuke]

For future, you may want to remember that formula as the pH of buffers.

 

[Borek]

For future, you may want to remember that formula as the pH of buffers.

No, these things are completely unrelated.

never mind.. I asked a classm8 and he told me about the pH=pka1+pka2 /2 equation.

Close, but wrong.

 

[DeeNos]

I would suggest first identifying the relevant equations and concepts needed to solve this problem. In this case, we can use the Henderson-Hasselbalch equation to calculate the pH of a solution containing a weak acid and its conjugate base. This equation is pH = pKa + log([A-]/[HA]), where pKa is the negative logarithm of the acid dissociation constant and [A-] and [HA] are the concentrations of the conjugate base and acid, respectively.

In this problem, we are dealing with a polyprotic acid, which means it can donate more than one proton. Therefore, we need to consider all three dissociation constants and their corresponding conjugate base and acid concentrations. We can use the mass balance equation to relate the concentrations of these species, as well as the charge balance equation to ensure that the solution is electrically neutral.

Once we have all the necessary equations, we can plug in the given values for the dissociation constants and the initial concentration of the acid, and solve for the pH using the Henderson-Hasselbalch equation. The resulting pH should be around 2.5 for a 0.1M Na2HAsO4 solution.

In summary, the key to solving this problem is to understand the relevant equations and concepts, and to carefully apply them to the given information. It may also be helpful to review the basics of acid-base chemistry and practice similar problems to gain a better understanding of the concepts involved.