[biospy]

What would be the pH of a solution prepared by combining equal quantities of NaH2PO4 and Na2HPO4? Explain with an equation.

K1=7.5*10^-3
K2=6.2*10^-8
K3=4.2*10^-13
(for phosphoric acid)

I am puzzled... would you use the K2 and take the -log of it?? I can't think of any other solution.....and I already used the K1 to find the pH for H3PO4 and H2PO4-, but the Na is throwing me off a bit and I don't know why....

[enahs]

Polyprotic acids can be complicated, here is some info on how to actually calculate them (from a regular forum user here site:)
http://www.chembuddy.com/?left=pH-calculation&right=pH-polyprotic-acid-base
Most of that probably looks really confusing to you, do not worry (that is FYI).

Do not let all that information scare you however. Chemistry is not math, before you do the math look at what you have and what you are trying to get.

Notice your K1, it is clearly of a weak acid. Notice your K2, it is of a even weaker acid.

In this case, because they are both weak acid steps, and the order of magnitude between then K1 and K2 (and K3) are large, we can make the assumption that it is a monoprotic acid and K_a = K1. And because the orders of magnitude are so great, this is an excellent assumption in this case. Another point of fact, with the number of significant figures you wrote, there will be absolutely no difference if you do it the "complicated but more complete" way in the site I linked to. If you can understand what the site is saying and can do it, do both ways and compare with the correct number of significant figures.

[Borek]

Na⁺ is just a spectator - don't let it add confusion, it doesn't do anything here.

What you have is a solution of H₂PO₄^- and HPO₄^2- - weak acid and its conjugate base. Does it ring a bell?

[english]

A little helpful equation:

pH = pKa + log [A-]
                         [HA]

[vhpk]

A little helpful equation:

pH = pKa + log [A-]
                         [HA]

This equation is called Henderson-Hasselbalch equation, it is used only for  buffer solution, we also have the same equation for base use pK_b

[Borek]

Henderson-Hasselbalch equation, it is used only for  buffer solution

Think it over - it holds for every solution containing pair of conjugated acid and base, doesn't matter if the solution can be classified as buffer or not.

[vhpk]

Henderson-Hasselbalch equation, it is used only for  buffer solution

Think it over - it holds for every solution containing pair of conjugated acid and base, doesn't matter if the solution can be classified as buffer or not.

Sorry for my false idea, do we use this equation for weak acid and base?

[Borek]

It is most convenient in the case of weak acids/bases, but in fact it can be used for every acid/base, as every acid/base has its own dissociation equilibrium and dissociation constant. Just in some cases it can be neglected, like for HCl - no matter what pH we have we can safely assume it is completely dissociated. This isn't so for HSO₄^2- which dominates sulfuric acid solutions with concentrations higher than about 0.01M

See http://www.chembuddy.com/?left=pH-calculation&right=pH-buffers-henderson-hasselbalch for more details and discussion.