# What is the S2− in 0.10 M hydrogen sulfide?

I found this question in one of the problem sets and I decided to try it, I keep getting 9.1 × 10−8 as the answer and I thought this since this is a diprotic acid and I made two equations and I keep getting that number.

Ka1: 1 × 10−7

Ka2: 9.1 × 10−8

Anyone know why? Here are the answer choices:

1.3 × 10−18

1.3 × 10−8

1.3 × 10−13

1.3 × 10−15

Why is my answer so different? For the first equation I get x = 1 × 10−4 and then for the second equation I get 9.1 × 10−8 all the time.

Please help.

• What are the relevant pKas? – jerepierre Feb 10 '15 at 22:44
• @jerepierre Gave the Ka1 and Ka2 – Asker123 Feb 10 '15 at 22:45
• What equation are you using to get x = 1E-4 and what is x? – jerepierre Feb 10 '15 at 22:47
• I am using the Ka ionization constant to get the molarities. – Asker123 Feb 10 '15 at 22:47
• The second Ka must be wrong. 9e-8 is pretty much the same as 1e-7, and it doesn't make sense that the second deprotonation is nearly the same as the first. Wikipedia has the second pKa as >14. – jerepierre Feb 10 '15 at 23:17

## 1 Answer

Your value for Ka2 is incorrect.

http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch17/diprotic.php

• Yeah good point the chart that I had was incorrect. Thanks for pointing that out. – Asker123 Feb 10 '15 at 23:21