When do we include the [H2O] in calculating the K value?

[golden]

[b^3]

When looking for K the equilibrium constant, you include [H₂O]
When looking for K_a the acidity constant, you don't include [H₂O]

EDIT: as K * [H₂O] =  K_a
[H₂O] will be a constant, hence we can do this.

[zool3]

i thought you never include water because the rule states that you never include substances that are solid or liquid

[Greatness]

i thought you never include water because the rule states that you never include substances that are solid or liquid

What do you mean by that?
If you're just finding the equilibrium constant then you include water in your fraction.
But when youre talking about Ka the acidity constant you dont include it cos it's already been taken into account.

[funkyducky]

I was under the assumption that you include [H2O] for steam/water vapour, as it actually has concentration, but technically speaking a liquid solvent doesn't have concentration, because concentration of an aqueous substance is by definition the amount of a substance dissolved in a certain volume of solvent. *shrug* Please enlighten me if/why I'm right/wrong.

[Mao]

I was under the assumption that you include [H2O] for steam/water vapour, as it actually has concentration, but technically speaking a liquid solvent doesn't have concentration, because concentration of an aqueous substance is by definition the amount of a substance dissolved in a certain volume of solvent. *shrug* Please enlighten me if/why I'm right/wrong.

This.

The quantity you are trying to express is called 'activity', which is an 'effective concentration'. Pure compounds (solids, liquids) have an 'activity' of 1, i.e. they do not have an 'effective concentration'.

[Graphite]

When looking for K the equilibrium constant, you include [H₂O]
When looking for K_a the acidity constant, you don't include [H₂O]

EDIT: as K * [H₂O] =  K_a
[H₂O] will be a constant, hence we can do this.

I agree with your Ka explanation. Since it is constant, we can ignore it as all Ka values are only relative to one another, a good way to eliminate 1 parameter.

@funkyducky I am a bit confused as to why you need to include [H2O] for steam (water in gas state)? So your saying that the steam has substances in it?

[funkyducky]

No, I'm saying that as a gas, like any gas, water has concentration, in that you can have a variable amount in a given volume.

[taqi]

on a kind of related note: if I add water to a reaction which has the same number of particles on both sides, but includes water (as a liquid) as a reactant, does the equilibrium position shift to the right?

[zool3]

i thought you never include water because the rule states that you never include substances that are solid or liquid

What do you mean by that?
If you're just finding the equilibrium constant then you include water in your fraction.
But when youre talking about Ka the acidity constant you dont include it cos it's already been taken into account.

i meant like you know when you're finding K you only use concentrations for substances that have a gaseous or aqueous states. not liquid or solid. if that makes anymore sense :S

[thushan]

on a kind of related note: if I add water to a reaction which has the same number of particles on both sides, but includes water (as a liquid) as a reactant, does the equilibrium position shift to the right?

In that case, you would think about an initial change in total concentration of aqueous species, rather than the amount of water.