Question 17.12: Predicting Precipitation Reactions by Comparing Q and Ksp A ...
Predicting Precipitation Reactions by Comparing Q and K_{sp}
A solution containing lead(II) nitrate is mixed with one containing sodium bromide to form a solution that is 0.0150 M in Pb(NO_3)_2 and 0.00350 M in NaBr. Does a precipitate form in the newly mixed solution?
First, determine the possible cross products and their K_{sp} values (Table 17.2). Any cross products that are soluble will not precipitate (see Table 8.1).
Possible cross products:
NaNO_3 soluble
PbBr_2 K_{sp}= 4.67 × 10^{-6}
Calculate \mathcal{Q} and compare it to K_{sp} . A precipitate only forms if \mathcal{Q} > K_{sp} .
\mathcal{Q}= [Pb^{2+}][Br^-]^2
= (0.0150)(0.00350)^2
= 1.84 × 10^{-7}
\mathcal{Q}< K_{sp} ; therefore, no precipitate forms.
| TABLE 17. 2 Selected Solubility-Product Constants (K_{sp}) at 25 °C | |||||
| Compound | Formula | K_{sp} | Compound | Formula | K_{sp} |
| Barium fluoride | BaF_2 | 2.45 × 10^{-5} | Lead(II) chloride | PbCl_2 | 1.17 × 10^{-5} |
| Barium sulfate | BaSO_4 | 1.07× 10^{-10} | Lead(II) bromide | PbBr_2 | 4.67 × 10^{-6} |
| Calcium carbonate | CaCO_3 | 4.96 × 10^{-9} | Lead(II) sulfate | PbSO_4 | 1.82 × 10^{-8} |
| Calcium fluoride | CaF_2 | 1.46 × 10^{-10} | Lead(II) sulfide* | PbS | 9.04 × 10^{-29} |
| Calcium hydroxide | Ca(OH)_2 | 4.68 × 10^{-6} | Magnesium carbonate | MgCO_3 | 6.82 × 10^{-6} |
| Calcium sulfate | CaSO_4 | 7.10 × 10^{-5} | Magnesium hydroxide | Mg(OH)_2 | 2.06 × 10^{-13} |
| Copper(II) sulfide* | CuS | 1.27 × 10^{-36} | Silver chloride | AgCl | 1.77 × 10^{-10} |
| Iron(II) carbonate | FeCO_3 | 3.07 × 10^{-11} | Silver chromate | Ag_2CrO_4 | 1.12 × 10^{-12} |
| Iron(II) hydroxide | Fe(OH)_2 | 4.87 × 10^{-17} | Silver bromide | AgBr | 5.36 × 10^{-13} |
| Iron(II) sulfide* | FeS | 3.72× 10^{-19} | Silver iodide | AgI | 8.51 × 10^{-17} |
*Sulfide equilibrium is of the type: MS(s) + H_2O(l) \rightleftharpoons M_{2+}(aq) + HS^-(aq) + OH^-(aq)
TABLE 8.1 Solubility Rules for Ionic Compounds in Water
| Compounds Containing the Following Ions Are Generally Soluble |
Exceptions |
| Li^+, Na^+, K^+, and NH^+_4 | None |
| NO_3^- and C_2H_3O_2^- | None |
| Cl^-, Br^-, \ and \ I^- | When these ions pair with Ag^+, Hg^{2+}, or Pb^{2+} , the resulting compounds are insoluble. |
| SO_4^{2-} | When SO_4^{2-} pairs with Sr^{2+} , Ba^{2+} , Pb^{2+} , Ag^+, or Ca^{2+}, the resulting compound is insoluble. |
| Compounds Containing the Following Ions Are Generally Insoluble |
Exceptions |
| OH^- and S^{2-} | When these ions pair with Li^+, Na^+, K^+, \ or NH^+_4, the resulting compounds are soluble. |
| When S^- pairs with Ca^{2+}, Sr^{2+}, or Ba^{2+}, the resulting compound is soluble. | |
| When OH^- pairs with Ca^{2+}, Sr^{2+},\ or Ba^{2+}, the resulting compound is slightly soluble. | |
| CO^{2-}_3 and PO^{3-}_4
|
When these ions pair with Li^+, Na^+, K^+, \ or NH^+_4, the resulting compounds are soluble. |