Common-Ion Effect - AP Chemistry

1. For $CaF_2(s)$, what happens to solubility when $NaF(aq)$ is added?

   Solubility decreases (common ion $F^-$). Added fluoride shifts dissolution equilibrium left.

2. Which ion is the common ion when $\text{CH}_3\text{COOH}(aq)$ is mixed with $\text{CH}_3\text{COONa}(aq)$?

   $\text{CH}_3\text{COO}^-$. Both solutions contain the acetate ion.

3. For $AgCl(s)$, what happens to solubility when $NaCl(aq)$ is added?

   Solubility decreases (common ion $Cl^-$). Added chloride shifts dissolution equilibrium left.

4. In $\text{AgCl}(s) \rightleftharpoons \text{Ag}^+ + \text{Cl}^-$, what happens to $[\text{Ag}^+]$ when $[\text{Cl}^-]$ is increased?

   $[\text{Ag}^+]$ decreases. To maintain constant $K_{sp}$, $[\text{Ag}^+]$ must decrease.

5. Which ion is the common ion when $NH_4Cl$ is added to a solution containing $NH_3$?

   $NH_4^+$. Both $NH_4Cl$ and protonated $NH_3$ contain ammonium ion.

6. What is the solubility-product expression for $\text{MX}(s) \rightleftharpoons \text{M}^+ + \text{X}^-$?

   $K_{sp}=[\text{M}^+][\text{X}^-]$. Product of ion concentrations at equilibrium.

7. Assuming $[\text{F}^-]_{\text{initial}} \gg 2s$, what is $s$ for $\text{CaF}_2$ in $[\text{F}^-]$ using $K_{sp}$?

   $s\approx\frac{K_{sp}}{[\text{F}^-]^2}$. When common ion dominates, solve $K_{sp}$ for cation concentration.

8. What is the solubility product expression for $MX(s)\rightleftharpoons M^+ + X^-$?

   $K_{sp}=[M^+][X^-]$. Product of ion concentrations for 1:1 salt dissolution.

9. Identify the direction of shift when $NaOH$ is added to $NH_3(aq)$.

   Shift left; $NH_3$ forms less $NH_4^+$. Added $OH^-$ shifts ammonia-water equilibrium left.

10. What happens to $pH$ when $A^-$ (the conjugate base) is added to a buffer of $HA/A^-$?

    $pH$ increases. More conjugate base consumes $H^+$, raising pH.

11. Identify the direction of shift when $CH_3COONa$ is added to $CH_3COOH(aq)$.

    Shift left; $CH_3COOH$ ionizes less. Added acetate ion suppresses acetic acid dissociation.

12. State the equilibrium expression for a weak base $B$ with $K_b$.

    $K_b=\frac{[BH^+][OH^-]}{[B]}$. Products over reactants for weak base protonation equilibrium.

13. Which relationship is correct for a conjugate pair: $K_a$ and $K_b$ at $25^\circ C$?

    $K_aK_b=K_w=1.0\times 10^{-14}$. Conjugate acid-base pairs multiply to water's ion product.

14. What is the net effect on $[\text{OH}^-]$ when $\text{BHCl}$ is added to a solution of weak base $\text{B}$?

    $[\text{OH}^-]$ decreases. Added $\text{BH}^+$ shifts equilibrium left, reducing base ionization.

15. Identify the common ion when $\text{CaF}_2(s)$ is placed in a solution of $\text{NaF}(aq)$.

    $\text{F}^-$. Fluoride ion is present in both the solid and added salt.

16. Which ion is the common ion when $NaF$ is added to a solution containing $HF$?

    $F^-$. Both $NaF$ and $HF$ contain fluoride ion in solution.

17. What is the net ionic equation for common-ion suppression of $HA$ by adding $A^-$?

    $HA(aq)\rightleftharpoons H^+(aq)+A^-(aq)$ shifts left. Added $A^-$ drives equilibrium toward undissociated $HA$.

18. State the equilibrium expression for a weak acid $HA$ with $K_a$.

    $K_a=\frac{[H^+][A^-]}{[HA]}$. Products over reactants for weak acid dissociation equilibrium.

19. What is the buffer relationship between conjugate pairs: $\text{HA}$ and $\text{A}^-$, in terms of the common-ion effect?

    $\text{A}^-$ suppresses ionization of $\text{HA}$. Conjugate base reduces conjugate acid ionization.

20. What is the net ionic equation for common-ion suppression of $B$ by adding $BH^+$?

    $B(aq)+H_2O(l)\rightleftharpoons BH^+(aq)+OH^-(aq)$ shifts left. Added $BH^+$ drives equilibrium toward unprotonated $B$.

21. Which ion is the common ion when $\text{NH}_3(aq)$ is mixed with $\text{NH}_4\text{Cl}(aq)$?

    $\text{NH}_4^+$. Both solutions contain the conjugate acid of ammonia.

22. What happens to $[OH^-]$ when a strong base is added to a weak base solution?

    $[OH^-]$ increases; weak base ionization decreases. Strong base fully dissociates, suppressing weak base's partial ionization.

23. What is the molar solubility $s$ of $\text{AgCl}$ in pure water in terms of $K_{sp}$?

    $s=\sqrt{K_{sp}}$. For 1:1 salt, $K_{sp}=s^2$ where $s$ is molar solubility.

24. Identify the common ion when $\text{BaSO}_4(s)$ is placed in a solution of $\text{Na}_2\text{SO}_4(aq)$.

    $\text{SO}_4^{2-}$. Sulfate ion is present in both the solid and added salt.

25. What is the equilibrium expression for a weak acid $\text{HA}$: $\text{HA} \rightleftharpoons \text{H}^+ + \text{A}^-$?

    $K_a=\frac{[\text{H}^+][\text{A}^-]}{[\text{HA}]}$. Products over reactants for weak acid dissociation.

26. What happens to $[H^+]$ when a strong acid is added to a weak acid solution?

    $[H^+]$ increases; weak acid ionization decreases. Strong acid fully dissociates, suppressing weak acid's partial ionization.

27. What is the net effect on $[\text{H}^+]$ when $\text{NaA}$ is added to a solution of weak acid $\text{HA}$?

    $[\text{H}^+]$ decreases. Added $\text{A}^-$ shifts equilibrium left, reducing acid dissociation.

28. Which direction does $\text{HA} \rightleftharpoons \text{H}^+ + \text{A}^-$ shift when $[\text{A}^-]$ is increased?

    Left (toward $\text{HA}$). Excess product drives equilibrium toward reactants.

29. What is the effect on molar solubility when a common ion is added to a sparingly soluble salt?

    Molar solubility decreases. Common ion shifts dissolution equilibrium left, reducing dissolved ions.

30. What happens to the molar solubility of $\text{AgCl}(s)$ when $\text{Cl}^-$ is added from $\text{NaCl}$?

    Molar solubility decreases. Common ion shifts dissolution equilibrium left.

31. What is the equilibrium expression for a weak base $\text{B}$: $\text{B}+\text{H}_2\text{O} \rightleftharpoons \text{BH}^+ + \text{OH}^-$?

    $K_b=\frac{[\text{BH}^+][\text{OH}^-]}{[\text{B}]}$. Products over reactants for weak base protonation.

32. What is the common-ion effect in aqueous equilibrium systems?

    Added common ion shifts equilibrium left, reducing dissociation/solubility. Excess product ions drive equilibrium backward per Le Châtelier's principle.

33. What is the molar solubility $s$ of $\text{CaF}_2$ in pure water in terms of $K_{sp}$?

    $s=\left(\frac{K_{sp}}{4}\right)^{\frac{1}{3}}$. For 1:2 salt, $K_{sp}=s(2s)^2=4s^3$.

34. Which direction does $\text{B}+\text{H}_2\text{O} \rightleftharpoons \text{BH}^+ + \text{OH}^-$ shift when $[\text{BH}^+]$ is increased?

    Left (toward $\text{B}$). Excess product drives equilibrium toward reactants.

35. Assuming $[\text{Cl}^-]_{\text{initial}} \gg s$, what is $s$ for $\text{AgCl}$ in $[\text{Cl}^-]$ using $K_{sp}$?

    $s\approx\frac{K_{sp}}{[\text{Cl}^-]}$. When common ion dominates, solve $K_{sp}$ for remaining ion.

36. What is the common-ion effect in aqueous equilibrium systems?

    Adding a shared ion shifts equilibrium left, reducing ionization/solubility. Le Châtelier's principle: system responds to minimize disturbance.

37. What is the solubility product expression for $M(OH)_2(s)\rightleftharpoons M^{2+}+2OH^-$?

    $K_{sp}=[M^{2+}][OH^-]^2$. Includes squared $[OH^-]$ due to stoichiometry coefficient of 2.

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