# Dissociation type problem

I think that is what you call it. I need to find the pH of a solution containing 50 grams of Na3PO4.

50 g ---- > 0.305 M

We know that Kw=Ka*Kb

So,

Na3PO4 + H20 --> HNa3PO4 + OH (I think)

Kb = x^2/(.305-x)

I cant find the value of Kb anywhere in my book, internet. If I had it I could calculate Ka and get my pH. Otherwise Im stumped.

dextercioby
Homework Helper
Do you mind if i'm asking you to write the dissociation relation correctly...?

Daniel.

MMM? Doent 10^-14 = Kw = Ka*Kb

and Na3PO4 + H20 --> HNa3PO4 + OH
(Base) (Acid) (Con A) (Con B)

Initial: .305M -- 0 0
Change -x -- +x +x
Final: .305 -x -- x x

Kb = products/reactants = [x][x] / [.305 - x]

If I knew what the value of Kb was for Na3PO4, I could get x and calculate the pOH and ofcourse get the pH.

dextercioby
Homework Helper
The chemical (well,ionic) reaction is incorrectly written...Think it viceversa:the base +acid--->Na phosphate+water.

Daniel.

The problem doesnt mention any specific acid and all the preceeding problems use water in the reactants. Anyhow, cant water be considered an acid (thats browsy rule or something)?

I thought the Base would pick up an H+ and since the H20 lost an H, it would simply become hydroxide

dextercioby
Homework Helper
I saw it like
$$Na_{3}PO_{4}+3H_{2}O\rightleftharpoons H_{3}PO_{4}+3NaOH$$

Anyway,pay attention with the solving and about the data that u're missing,i don't know,maybe someone else would guide u better...

Daniel.

Gokul43201
Staff Emeritus
Gold Member
The hydrolysis may not be complete, and so you might also have Na2HPO4 and NaH2PO4. From thie equilibrium constants you can tell which one (if any) is dominant.

GCT
Homework Helper
In cases such as these you observe the corresponding cationic and anionic components of the salt, the pH will differ according to whether the conjugates of such are strong acid/bases. In this case the base will contribute to the pH. Usually for these types of problems, we can neglect matters regarding the formation H2PO4 since the conjugate base of it is a fairly weak base.

what's this?
50 g ---- > 0.305 M
Why don't you try stating the problem word by word.

#80.

Trisodium phosphate (Na3PO4) is available in hardware stores as TSP and used as a cleaning agent. The label on a box of TSP warms that the substance is very basic. What is the pH of a solution containing 50.0 g of TSP per liter?

Gokul43201
Staff Emeritus
Gold Member
Do you see why it's important to not leave out the "per liter" ?

GCT
Homework Helper
I suppose that they want us to assume that it is a strong base

-The net equation will consist of aqueous $$PO_4^-3$$ reacting with a hydronium cation $$H_3O^+$$ to produce $$HPO_4^-2$$ and $$H_2O$$ Write and balance out this equation.

-You can than deduce the final concentration of $$OH^-$$ from finding the initial molarity of the $$PO_4^-3$$, there is a stochiometric one to one equivalence of the compound to this anion, so find the initial molarity of the compound and this will be the concentration of the anion.

-Use the balanced equation to find the stoichiometric ratio of the anion to the hydroxide.

-Use this to find the $$p_{OH}$$ then deduce $$p_H$$

Borek
Mentor
GeneralChemTutor said:
The net equation will consist of aqueous $$PO_4^-3$$ reacting with a hydronium cation $$H_3O^+$$ to produce $$HPO_4^-2$$ and $$H_2O$$.
And why not $$PO_4^3^- + H_2O \rightarrow HPO_4^2^- + OH^-$$?

Due to a very small third dissociation constant you may assume that the equilibrium is shifted 100% to the right - the you may proceed as you have proposed, but this way you don't have to introduce $$H_3O^+$$ ions.

Chemical calculators for labs and education
BATE - pH calculations, titration curves, hydrolisis

GCT
That is the base increases the pH level by first reacting with the strongest acid , the $[H^+]$ level decreases as a result. You can guess what happens next.