You are here:

Chemicals/what is "activity"?

Advertisement

Expert: - 11/26/2005


Question
Hello,
I was reading about the equilibrium constant for the self ionization of water K=[H30+][OH-] and the discussion mentioned that we do not include the concentration of water in this expression for dilute solutions since [H2O] is practically constant at about 55.5M and its activity equal to 1.

What does that phrase "activity equal to 1" mean?

Thanks for your time,
Mark

Answer
Activity is really the mole fraction of each component (compared to total = 1) available for the equilibrium in question.  In ideal dilute solutions, the activity of things like H+ and OH- equals the concentration in question.  For the H2O, that mole fraction is equal to 1 minus the mole fractions of the H+ and OH-, which are very small, so the activity of water is approximately 1 [1 - 10(-7)].

When we do calculations for dilute solutions, we just use the concentrationsince they equal the activity.  Activity becomes very important in concentrated solutions.

A detailed explanation.

http://www.lsbu.ac.uk/water/activity.html

Chemicals

All Answers

Answers by Expert:

Ask Experts

Volunteer

Henry Boyter

Expertise

No homework or other schoolwork! The question will be rejected. If you have not searched www.google.com, do so before posting. If you are a student, give your grade and course. Everyone, explain the purpose and context for the question.
Experience in the area
Chemistry (non-biochemistry), environmental science, occupational health and safety, environmental regulation and management, environmental engineering, and wastewater engineering. I'm the Director of Environmental, Health, and Safety and the Director of Research at the Institute of Textile Technology.

Experience

Chemistry (non-biochemistry), environmental science, occupational health and safety, environmental regulation and management, environmental engineering, and wastewater engineering. I'm the Director of Environmental, Health, and Safety and the Director of Research at the Institute of Textile Technology.

©2012 About.com, a part of The New York Times Company. All rights reserved.