What a learner can do afterwards
- Calculates ionic strength for a mixed electrolyte and explains why charge is squared in it
- Applies the limiting law to obtain a mean activity coefficient and states its range of validity
- Explains why a measured pH drifts from the calculated value as salt is added
- Says why single-ion activities cannot be measured and what is quoted instead
1 · Read
Ions in solution gather a cloud of opposite charge, the ionic atmosphere, which shields each ion. Ionic strength measures how crowded that world is: halve the sum of concentration times charge squared. Charge enters squared because an ion's charge both builds its atmosphere and feels it, so a doubly charged ion counts four times as much as a singly charged one.
For 0.10 M sodium chloride both charges are 1, so I is half of (0.10 + 0.10), which is 0.10 M. For 0.10 M magnesium sulphate both charges are 2, so I is half of (0.10 x 4 + 0.10 x 4), which is 0.40 M.
The Debye-Huckel limiting law gives the mean activity coefficient: log10 of gamma equals -0.509 times |z+ z-| times the square root of I, for water at 25 C. For 0.010 M sodium chloride, log gamma is -0.0509, so gamma is about 0.89. It holds only in very dilute solutions. Extra salt reshapes every ion cloud, so a measured pH drifts even when no acid or base was added.
No experiment can isolate one ion type, so single-ion activities cannot be measured. Tables quote mean coefficients for the whole salt instead.
Charge squared sets the strength, the cloud lowers the activity, and only whole-salt means ever reach print.
The rewritten readings were written for this dataset. Reading '11.2: Electrolytes' remains adapted from OpenStax Chemistry 2e (CC BY-NC-SA 4.0), openstax.org.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.