Electrolyte Solutions and the Debye-Huckel Limiting Law · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
En anglais

Ions never sit alone

Science · Chemistry · ages 20-22
Name ______________________   Date ____________

  1. Why is charge squared in ionic strength?

    • Because squaring keeps every value positive
    • Its charge both builds and feels the cloud
    • Because concentrations must always be squared

  2. How is ionic strength defined?

    • Half the sum of concentration times charge squared
    • The total mass of dissolved salt
    • The pH of the solution

  3. The Debye-Huckel limiting law holds only in very dilute solutions.

    Circle one:   True   False

  4. Potassium nitrate is added to a buffer, and the measured pH shifts although no acid or base was added. Why?

    • Nitrate always destroys every buffer instantly
    • pH meters fail whenever any salt is present
    • Added ions shift the activities

  5. A solution is 0.20 M sodium chloride. What is its ionic strength in M? Type the number.

    Answer: ______________

  6. Why are single-ion activities never quoted in tables?

    • Because only neutral combinations can be measured
    • Because single ions do not exist in water
    • Because tables have no room for extra rows

  7. A solution is 0.10 M magnesium chloride. What is its ionic strength?

    • 0.10 M
    • 0.20 M
    • 0.30 M

  8. A student applies the limiting law at 1.0 M and trusts the result. What is wrong?

    • The law fails for every salt containing sodium
    • The law only holds in very dilute solutions
    • Mean coefficients cannot be calculated at any concentration
LightMySky · lightmysky.comW1-mt_6wAcvWy0dv-s1

Answer key

For grown-ups. Fold this page away before handing over the rest.

Ions never sit alone W1-mt_6wAcvWy0dv-s1

  1. Its charge both builds and feels the cloud · The charge enters twice, once building the cloud and once feeling it, so the weight goes as charge squared.
  2. Half the sum of concentration times charge squared · Both amount and charge matter, with charge weighted heavily.
  3. True · Crowded clouds break the simple picture the law assumes.
  4. Added ions shift the activities · Ionic strength moved, so the effective concentrations moved with it.
  5. 0.2 · Half of 0.20 times 1 plus 0.20 times 1 is 0.20.
  6. Because only neutral combinations can be measured · Every measurement sees whole salts, so tables quote whole-salt means.
  7. 0.30 M · Half of 0.10 times 4 plus 0.20 times 1 is 0.30 M.
  8. The law only holds in very dilute solutions · At 1.0 M the clouds crowd and the simple picture collapses.
Worksheet · LightMySky