# Homework Help: Monoprotic weak acid in water

1. Nov 24, 2015

### yango_17

1. The problem statement, all variables and given/known data
If enough of a monoprotic acid is dissolved in water to produce a 0.0153 M solution with a pH of 6.31, what is the equilibrium constant, Ka, for the reaction?
2. Relevant equations

3. The attempt at a solution
I have attempted to solve this problem using ICE tables, but keep getting the incorrect answer. One feedback stated that I needed to account for the initial H+ concentration due to water, which is 10^-7 M, but I'm not sure how to go about putting this into the ICE table. Thanks in advance for any help

2. Nov 24, 2015

### Staff: Mentor

Please show your work and the result you got, hard to comment on the problem not seeing what you did.

In general concentration of H+ from the water autodissociation is not 10-7 M, as its concentration changes when the acid dissociates shifting the water dissociation equilibrium. This can be still solved, but not with just an ICE table. You would need a full, systematc approach, which requires writing all equations describing the system (Ka, Kw, mass balances, charge balance) and solving for Ka.

3. Nov 25, 2015

### epenguin

Instead of tables which IMHO will leave you with no knowledge or ability a year after the last time you've used them, I would use the simple fundamentals which you will be able to call on any time needed.

What are the relevant equations for this question?