[sun725]

Hi,

I want to calculate the pH for Na2SeO3 and Na2SeO4 , however I'm not sure about the method. Should I use an ICE-table here? Or simply plug the given concentration to -log ..?

I hope I can get an explanation.

Thanks in advance.

[Borek]

Start with the reaction equations, that should give you a hint.

Note that each method you have mentioned has some limits of applicability - do you know them? They should tell you which one will work here.

Not to mention the fact they are all just simplified solutions of the more general case.

[sun725]

Hi,

Thanks for answering ..

So for sodium salenite, it should be: SeO2 + 2 NaOH → Na2SeO3 + H2O ..?
So they consists of a strong base and a weak acid?

I don't know how to move on from here.. 

[Borek]

No.

What you wrote is the salt synthesis, and you are asked to calculate pH of a salt solution. Why is the pH of the salt solution different from neutral? Salt per se doesn't contain any H⁺ nor OH^-, so why (and how) does it change pH?

[sun725]

So these are salts of a strong base (NaOH) and a weak acid, e.g. H2SeO3 and H2SeO4, right?

Back to the equation, I guess it should be for Na2SeO3
SeO3^2-+H2O  HSeO3^-+OH^-

What should be a next move here?

Thanks again

[Borek]

SeO3^2-+H2O  HSeO3^-+OH^-

What should be a next move here?

Calculation of the OH^- from Ka (actually Kb).

Please show some effort - after each small step you ask "what next?". That's asking for a spoon feeding and that's not what we do here. I have already pointed you to the salt hydrolysis, google for ways of calculating pH of such solutions, or consult your notes or a textbook.

[AWK]

H₂SeO₄ is a strong acid.

[sun725]

Hi,

I'm very sorry if I "asking for at a spoon feeding", however this is very new to me and believe me I want to make an effort..

I found the pKa values for HSeO4- and HSeO3- (there is also H2SeO3) and I'm confused of which one to use.

I can't seem to find a Kb value anywhere on my book.

[AWK]

K_b is very easily calculable. Check any texbook concerning acid and bases.

[sun725]

Okay,

I calculated the Kb from the pH I measured for both Na2SeO3 (pH:9.84) and Na2SeO4 (pH:9.96) and I have to find the theoretical pH.

[OH-]=10^-pOH

I got Kb for  Na2SeO3 to be: 4.8*10^-8

And for Na2SeO4: 8.3*10^-8

Is that correct? And please no derogatory comments. I really try my best.

Thanks in advance.

[AWK]

Without known concentration you calculations are worthless.
Read something about acid-base equilibrium (textbook, wikipedia or
http://www.chembuddy.com/?left=pH-calculation&right=toc).
Both your pH values concern rather solutions of Na₂SeO₃.
For Na₂SeO₄ pH should be a bit higher than 7.

[sun725]

I thought I wrote the concentration earlier, however it's 0,1 for both Na2SeO3 and Na2SeO4

[AWK]

I got Kb for  Na2SeO3 to be: 4.8*10-8

Kb is for acids and bases not for salts. Selenous acid shows two K_b values.
Show data you used since this value for K_b1 of SeO₃^2- is rather wrong. For number from wikipedia the K_b1 value is of order 10^-7.