[ghostanime2001]

Q. What concentration of H₂Se is required to produce a pH of 1.93 ?
A. 0.81 kmol/m^3

H₂Se HSe^- + H⁺

10^-1.93 = H⁺
0.0117 M = H⁺

Since H₂Se is a weak acid, I need to make I.C.E table.
The equilibrium concentration of H₂Se is x-0.0117.
So the original concentration of H₂Se must be x.

I do not have the K_a provided for H₂Se but many internet sources say the pKa is 3.89 which translates to 1.29 x 10^-4.
Substituting into the eq expression, I get:

x = 1.07

which doesn't match the answer in the worksheet. What am I doing wrong ?

*btw how do you typset fractions and tables in this forum?

[Borek]

which doesn't match the answer in the worksheet. What am I doing wrong ?

I can confirm 1.07M gives pH 1.93, assuming H₂Se is a monoprotic acid with a given pKa. Neither of these assumptions has to hold.

[ghostanime2001]

Neither of these assumptions has to hold.

What do you mean by this ? and yes I did assume only K_a1 the first dissociation not the second. However I found another website which lists the K_a of hyrogen selenide as 1.7 x 10^-4. However, I checked in the CRC handbook of chemistry and physics that the pKa of hydrogen selenide is 3.89 which results the same Ka as before not the new one. I don't understand which is right.

[Borek]

Neither of these assumptions has to hold.

What do you mean by this ?

Without knowing what the pKa value was used by the book authors you can't never be sure if the value you used for calculations is the same they used - so there is a possibility there is a difference between their result and your result. And not knowing what is the pKa2 value, you can't be sure it can be ignored.

It is not your fault - if they give an answer that you have to reproduce, they should also list input data used for calculations.

[ghostanime2001]

the pKa₂ value for H₂Se is 11 by the way.

[Borek]

the pKa₂ value for H₂Se is 11 by the way.

Which means it can be safely ignored. Do you know why?

http://www.chembuddy.com/?left=pH-calculation&right=pH-polyprotic-simplified

[ghostanime2001]

very low proton concentration resulting from the dissociation of HSe^- so i guess the first dissociation of H₂Se gives measurable amount of acidity ?

[Borek]

very low proton concentration resulting from the dissociation of HSe^-

Yes.

so i guess the first dissociation of H₂Se gives measurable amount of acidity ?

I suppose you mean that "only" first dissociation step is responsible for the observed acidity. It can be put that way.