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Number expressing the ratio of a substance's chemical activity to its molar concentration (seemole). The measured concentration of a substance may not be an accurate indicator of its chemical effectiveness as represented by the equation for a particular reaction; in such cases, the activity is calculated by multiplying the concentration by the activity coefficient. In solutions, the activity coefficient is a measure of how much the solution differs from an ideal solution.
This entry comes from Encyclopædia Britannica Concise. For the full entry on activity coefficient, visit Britannica.com.