[mama]

My question is that why we do not write water while find the ionization constant of an acid.

Prompt reply will be highly appreciated.

Regards

[Borek]

We assume it is constant so it can be moved into Ka.

[cliverlong]

We assume it is constant so it can be moved into Ka.

This has always intrigued me

1. How do we demonstrate this is a reasonable approximation ?

2. Why do we bother with this approximation? Why don't we just leave concentration of water in the K_c equation?

Thanks

Clive

[Borek]

1. Check for yourself - what is the starting concentration of water? Howm much does it change in 100% dissociated 0.1 M solution? If concentration is higher, ionic strength of solution is source of much larger errors.

2. Reverse the question - if concentration of water is for all practical purposes constant, why bother with an additional value in the equation, when it can be moved into the constant?