Equilibrium Constant and Activities

[Jasmeet Dhaliwal 3E]

I recall Dr. Lavelle in lecture saying that when entering concentrations (or partial pressures) into the equilibrium expression, we use the numbers without the units. The book introduces the concept of activities, which are unitless numbers that take into account deviations from ideal behavior. So when we enter quantities into equilibrium expressions, are we really just putting in their activities? And because no gas is ideal, how would we determine the extent to which the activity differs from the true quantity?

[Nikhil Davuluri 2A]

I'm pretty sure the book says that you multiply the concentrations by an activity coefficient. However, since we are treating everything as ideal the activity coefficient for all substances is 1. Therefore, the numbers work out the same and we just have to drop the units (the activity coefficient would usually cancel them out).
Hope this helps.