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# The pH of a 0.020 mol/L solution of H2S(aq) is?

### 1 Answer

- pisgahchemistLv 712 months agoFavorite Answer
What is the pH.......

Assume H2S is a monoprotic acid. The ionization of HS- is negligible. Suflide ions are only found in extremely basic solutions (an excess of OH- ions).

H2S(aq) <==> H+ + HS- ................. Ka = 9.5x10^-8

0.020M ...........0 .......0 .................... initial

-x ................... +x .... +x .................. change

0.020-x ........... x ...... x ................... equilibrium

Ka = [H+][HS-] / [H2S]

9.5x10^-8 = x^2 / (0.020-x) ............. assume 0.02-x = 0.2 since x will be small

x = 4.4x10^-5

[H+] = 4.4x10^-5M

pH = -log[H+] = 4.36 .......... to two significant digits

When taking the log of a number, only the digits to the right of the decimal reflect the precision in the original number.

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The answer is accurate. Don't be put off by the "thumbs down." It's the work of a troll.

Hmmmm? Is the troll just being a troll, or is the troll put off by the fact that H2S acts as a monoprotic weak acid? If the troll has a concern then he should speak up.