Excluding the molarity of H2O in K

[Sean1F]

So when we're calculating equilibrium constants, how do we know when to not include water? I know that under some circumstances, H2O will end up being in the numerator and denominator of the equation, but is with any equation that involves H2O, any equation with aqueous solutions on both sides, or will it be given?

[Henry_2A]

Hi!
You can look at the state of H2O. If it's liquid, do not include it in the equilibrium constant. If it's a gas or an aqueous, include it.

I hope this helps

[Alyssa Ly 2G]

Hello!

When writing the expression for the equilibrium constant K, in general, you don't include any solids or liquids. Thus, include H2O in the expression for K as long as the H2O isn't a solid or liquid in the chemical reaction.

[Jonathan Shyu 3L]

When calculating K, typically you do not include solids such as salts (e.g. NaCl) or liquids such as water (H2O). Unless H2O is in gas form, you do not include it.

[Abu Zhang 2D]

Just wondering, when H2O is involved in the reaction in its gaseous state, should we use molarity or partial pressure for calculation?

[Ashley Wilson 2L]

Yes, you would include H2O in the equilibrium constant if it was a gas. This is because it would have a concentration or partial pressure. Solids and liquids are not included in K because their change in concentration is negligible.

[indigoaustin 3H]

When water is a liquid, it is considered in excess and therefore would cancel out of K. When water is a gas, it should be included in the expression for K. When K is a solid, it should not be included in the K expression because we never include solids. Hope this helps!

[QUEP 2F]

Hi!

When calculating K, we don’t include any solids or liquids because their change in concentration is negligible, so you should only include H2O if it’s in its gaseous form.