[katherinetran25]

When is it appropriate to exclude or include the concentration of H2O from calculations for K or Q?

I have been asked to calculate the Free Energy produced from a reaction, for the hydrolysis of ATP, with the reaction:
ATP + H2O --> ADP + phosphate

Of course, since [H2O]=55mol/L, the calculations for free energy are significantly different if I exclude the water concentration. 

I am not sure if this is relevant, but at our level, my professor told us to disregard activity concentrations. My thinking is that I would include the H2O concentration, as we were given the molarity (mol/L) of each of the substituents in the reaction, so to keep it consistent, the water concentration should be included as well.
Further, since we are specifically told to "ignore" activities, I can't just disregard H2O's contribution based on its molar ratio being 1.

Thank you for any clarification.

[Corribus]

The activity of a pure liquid or solid is always approximated as equal to 1, because the effective concentration of a pure liquid or solid does not change appreciably as a function of temperature, pressure, volume, etc. So, you should be using 1 in your equilibrium expressions, not the molarity of pure water. For other substances, the activity is usually approximated as the molar concentration of the substance.

[Irlanur]

it just depends on which kind of equilibrium constant you want to have. usually the most appropriate one is the one that uses activities.