[mooreso]

Am taking an engineering design class after not having chemistry for 15 years.  I am having problems solving what appears to be a simple dissociation problem:

The pH of an acid mine drainage through which hydrogen sulfide bubbles continuously is measured and found to have a value of pH = 3.7.   Estimate the Concentration of S^2- in this water.

Answer: 3 x 10^-15 mol/L

Can anyone give me a hand so I can see the process for solving this? 

[Borek]

Not enough data to calculate concentration.

You need to use second overall dissociation constant (see dissociation constants explained), but without information about H₂S concentration you can't do anything.

[mooreso]

I taslked to the professor... he intimated that there is a solution and that somehow it involves assumptions pertaining to the initial concentration.  I agree with you, however, that without the initial concentration of H₂S it would eem impossible.

[lemonoman]

The pH of an acid mine drainage through which hydrogen sulfide bubbles continuously

without information about H₂S concentration you can't do anything.

There is no initial concentration...the water is 'saturated' with H₂S.

You need to use second overall dissociation constant

Definitely.  And the first overall dissociation constant.

This, and the K_sp of H₂S (probably also available in the textbook where you find the dissociation constant) ... that is all one needs (in addition to what is given).

[Borek]

without information about H₂S concentration you can't do anything.

There is no initial concentration...the water is 'saturated' with H₂S.

And that's information about concentration - the one that is needed - but it is still not quantitative, only qualitative.

You need to use second overall dissociation constant

Definitely.  And the first overall dissociation constant.

You can safely ignore first dissociation step.

http://www.chembuddy.com/?left=pH-calculation&right=pH-polyprotic-acid-base#eq9.13

See equation 9.13 - denominator in acidic solutions is for all practical purposes only pH dependent, with both dissociation constants being very low.

This, and the K_sp of H₂S (probably also available in the textbook where you find the dissociation constant) ... that is all one needs (in addition to what is given).

K_sp?

[lemonoman]

K_sp?

Solubility product constant.

For A₂B_(s) <--> 2 A⁺_(aq) + B^-_(aq)

K_sp = [A⁺_(aq)]²[B^-_(aq)] / [AB_(s)]
~~ [A⁺_(aq)]²[B^-_(aq)]

since [AB_(s)] ~ 1

where [] are activities, but can safely be approximated by the concentrations for dissolved substances.

[AWK]

Am taking an engineering design class after not having chemistry for 15 years.  I am having problems solving what appears to be a simple dissociation problem:

The pH of an acid mine drainage through which hydrogen sulfide bubbles continuously is measured and found to have a value of pH = 3.7.   Estimate the Concentration of S^2- in this water.

Answer: 3 x 10^-15 mol/L

Can anyone give me a hand so I can see the process for solving this? 

The simple approximate solution of this problem (assuming only H2S is responsible for acidity) - [H+]=K_a2

[Borek]

K_sp?

Solubility product constant.

What does it have to do with the pH of dissolved gas calculations?

[Borek]

The simple approximate solution of this problem (assuming only H2S is responsible for acidity) - [H+]=K_a2

What a shame I am explaining it on my own site...

http://www.chembuddy.com/?left=pH-calculation&right=pH-polyprotic-simplified#eq10.8

[lemonoman]

What does it have to do with the pH of dissolved gas calculations?

I made a small error, but I still think you're wondering about how pH ties into it:

For H₂S:

H₂S <--> 2 H⁺ + S^2-

Ksp = [H⁺]²[S^2-] / [H₂S]
So [S^2-] = (K_sp)[H₂S] / [H⁺]²

where [H⁺] = 10^-pH

My mistake was, the activity of H2S isn't 1 because (silly me) it's a gas.  And gases are more complicated because they dissolve in liquids AS a gas, rather than dissociating into ions.

So in conclusion, my way doesn't work.

[Borek]

What you call K_sp is just an overall second dissociation constant.