Destroyer acid/base/neutral problem

[datdentist]

---

Members don't see this ad.

A solution of Na2HPO4 is:
a) acidic
b) basic
c) neutral

Given for H3PO4:
K1 = 7.5 x 10^-3
K2 = 6 x 10^-8
K3 = 1 x 10^-12

Ok so I don't understand the explanation in destroyer for this problem. I knew that I needed to hydrolyze the compound (not including Na since it's a spectator), but after that I'm a little lost...

 

[DAT Destroyer]

A solution of Na2HPO4 is:
a) acidic
b) basic
c) neutral

Given for H3PO4:
K1 = 7.5 x 10^-3
K2 = 6 x 10^-8
K3 = 1 x 10^-12

Ok so I don't understand the explanation in destroyer for this problem. I knew that I needed to hydrolyze the compound (not including Na since it's a spectator), but after that I'm a little lost...

This is a tough one indeed.......However......doing a hard problem may pay huge dividends in the end. Most salts are easy to tell if acidic, basic or neutral using the methods I have shown in previous problems.

This one is tough, since Ka values are given. The trick is to decide if this anion.....HPO4-- is going to act as an acid or a base. Thus we must compare the Ka to the Kb. If acting as an acid.....it loses its final proton.....thus we are dealing with the Ka3 yes ? The base reaction of this will give you the Kb2. Look at how I set this up. If you can see this......simply compare how the Ka compares to the Kb. Since the Kb is larger......than the Ka...the solution of this salt will be basic.

Hope this helps.

Dr. Romano

 

[datdentist]

I understand that K1 and K2 are Kas by pH = -logKa, but I thought K3 would be a Kb? Is it considered a Ka since the other values are representative of acids? That's the only thing confusing me.

 

[DAT Destroyer]

I understand that K1 and K2 are Kas by pH = -logKa, but I thought K3 would be a Kb? Is it considered a Ka since the other values are representative of acids? That's the only thing confusing me.

K3 is the acid constant for the third ionization step. When I get a chance I will write down all the steps you need to know to illustrate the ionization process.

Dr Romano

 

[DAT Destroyer]

K3 is the acid constant for the third ionization step. When I get a chance I will write down all the steps you need to know to illustrate the ionization process.

Dr Romano

 

[cacajuate]

That really clarified that problem for me.

 

[rdub]

K3 is the acid constant for the third ionization step. When I get a chance I will write down all the steps you need to know to illustrate the ionization process.

Dr Romano

How do we know to assume that the K1, K2, and K3 values are Ka rather than Kb? Is it because you say "given for H3PO4"?

 

[DAT Destroyer]

How do we know to assume that the K1, K2, and K3 values are Ka rather than Kb? Is it because you say "given for H3PO4"?

Yes,,,,that is correct.