AP Chemistry Questions on Predicting Precipitation

📚 Quick Study Guide

• 🧪 Solubility Rules: These are general guidelines that tell you whether a compound will dissolve in water. Memorize key rules (e.g., all nitrates are soluble).
• ⚖️ Solubility Product Constant ($K_{sp}$): This value represents the equilibrium constant for the dissolution of a solid in water. A smaller $K_{sp}$ indicates lower solubility. The equation for $K_{sp}$ for a generic salt $A_xB_y$ is $K_{sp} = [A^{+y}]^x[B^{-x}]^y$.
• 🤔 Ion Product (Q): Q is calculated the same way as $K_{sp}$, but using initial concentrations instead of equilibrium concentrations.
• 💡 Comparing Q and $K_{sp}$:
  • If $Q < K_{sp}$, the solution is unsaturated and more solid can dissolve. No precipitation occurs.
  • If $Q = K_{sp}$, the solution is saturated and the system is at equilibrium.
  • If $Q > K_{sp}$, the solution is supersaturated and precipitation will occur until $Q = K_{sp}$.
• 📝 Predicting Precipitation: Calculate Q based on the initial ion concentrations. Compare Q to the $K_{sp}$ value provided.

Practice Quiz

1. Which of the following compounds is generally considered insoluble in water according to solubility rules?

   1. A. $NaNO_3$
   2. B. $KCl$
   3. C. $AgCl$
   4. D. $Li_2SO_4$

2. The solubility product constant ($K_{sp}$) for $AgCl$ is $1.8 \times 10^{-10}$. What is the molar solubility of $AgCl$ in pure water?

   1. A. $1.8 \times 10^{-10}$ M
   2. B. $9.0 \times 10^{-5}$ M
   3. C. $1.34 \times 10^{-5}$ M
   4. D. $3.6 \times 10^{-20}$ M

3. A solution contains $[Ag^+] = 2.0 \times 10^{-5}$ M and $[Cl^-] = 3.0 \times 10^{-6}$ M. Will a precipitate of $AgCl$ form? ($K_{sp}$ of $AgCl = 1.8 \times 10^{-10}$)

   1. A. Yes, because Q > $K_{sp}$
   2. B. No, because Q < $K_{sp}$
   3. C. Yes, because Q < $K_{sp}$
   4. D. No, because Q > $K_{sp}$

4. What is the correct $K_{sp}$ expression for the dissolution of $PbCl_2(s)$ in water?

   1. A. $K_{sp} = [Pb^{2+}][Cl^-]$
   2. B. $K_{sp} = [Pb^{2+}][2Cl^-]$
   3. C. $K_{sp} = [Pb^{2+}][Cl^-]^2$
   4. D. $K_{sp} = [Pb^{2+}]^2[Cl^-]^2$

5. If a solution contains $0.01$ M $Pb^{2+}$ and $0.01$ M $Cl^-$, will $PbCl_2$ precipitate? The $K_{sp}$ of $PbCl_2$ is $1.6 \times 10^{-5}$.

   1. A. Yes, because Q > $K_{sp}$
   2. B. No, because Q < $K_{sp}$
   3. C. The solution is at equilibrium.
   4. D. More information is needed.

6. Which of the following actions will NOT affect the solubility of $AgCl$?

   1. A. Adding NaCl
   2. B. Adding $AgNO_3$
   3. C. Adding $NH_3$
   4. D. Adding $KNO_3$

7. The $K_{sp}$ of $CaF_2$ is $3.9 \times 10^{-11}$. What is the concentration of $Ca^{2+}$ in a saturated solution of $CaF_2$?

   1. A. $3.9 \times 10^{-11}$ M
   2. B. $1.95 \times 10^{-11}$ M
   3. C. $2.2 \times 10^{-4}$ M
   4. D. $3.4 \times 10^{-4}$ M