Question

Hydrogen sulfide (H2S) is an odorous gas that can be removed from solution via gas stripping: H2S ⇔ H^(+) + HS^(-)

Ka =8.6x10^(-8)

Find the fraction of hydrogen sulfide in H2S form at pH 6 and pH 8.

Please show all steps and possible unit conversions.

Answer 1

H2S <-> H+ + HS-

Ka1 = 8.6*10^-8

apply:

buffer equation for pKa1

pKa1 = -log(Ka1) = -log(8.6*10^-8) = 7.065501

then, for a buffer:

H2S <-> HS- + H+

pH = pKa + log(HS- / H2S)

for pH = 6...

6= 7.065501 + log(HS- / H2S)

solve

10^(6-7.065501 ) = HS- / H2S

HS- / H2S= 0.0860001

So... in terms of H2S

0.0860001H2S =  HS-

for 1 mol of H2S --> 0.0860001*1 = 0.0860001 mol of HS-

fraction = 1 / (1+0.0860001) = 0.92081

apply the same for pH = 8, expect now much lower H2S

for pH = 8...

8 =  7.065501 + log(HS- / H2S)

solve

10^(8-7.065501 ) = HS- / H2S

HS- / H2S= 8.6

So... in terms of H2S

8.6=  HS-

for 1 mol of H2S --> 8.6*1 = 8.6 mol of HS-

fraction = 1 / (1+8.6) = 0.1041