# common ion effect on solubility

This is the common ion effect. The common ion effect suppresses the ionization of a weak base by adding more of an ion that is a product of this equilibrium. CC BY-SA 3.0. http://en.wiktionary.org/wiki/limestone [ "article:topic", "clark", "authorname:clarkj", "showtoc:no" ], Former Head of Chemistry and Head of Science, Pressure Effects On the Solubility of Gases, Common Ion Effect with Weak Acids and Bases. What is the solubility at 25°C of calcium fluoride (CaF2): (a) in pure water; (b) in 0.10 M calcium chloride (CaCl2); and (c) in 0.10 M sodium fluoride (NaF)? The common-ion effect is used to describe the effect on an equilibrium involving a substance that adds an ion that is a part of the equilibrium. (b) Here the calcium ion concentration is the sum of the concentrations of calcium ions from the 0.10 M calcium chloride and from the calcium fluoride whose solubility we are seeking: Can we simplify this equation? Calculate the molar solubility of lead thiocyanate in 0.900 M KSCN. Whenever a solution of an ionic substance comes into contact with another ionic compound with a common ion, the solubility of the ionic substance decreases significantly. The solubility equilibrium constant can be used to solve for the molarities of the ions at equilibrium. If more concentrated solutions of sodium chloride are used, the solubility decreases further. Legal. Express the molar solubility numerically. limestoneAn abundant rock of marine and fresh-water sediments; primarily composed of calcite (CaCO₃); it occurs in a variety of forms, both crystalline and amorphous. Calculate concentrations involving common ions. Adding a common ion to a dissociation reaction causes the equilibrium to shift left, toward the reactants, causing precipitation. $$\mathrm{KCl \rightleftharpoons K^+ + {\color{Green} Cl^-}}$$ The equilibrium constant remains the same because of the increased concentration of the chloride ion. With such a small solubility product for CaF2, you can predict its solubility << 0.10 moles per liter. Wiktionary $CaF_2 \leftrightarrow Ca^{2+} + 2F^-$, (a) If the solubility in pure water is s, then, $K_{sp} = {[Ca^{2+}]}{[F^-]}^2$. Watch the recordings here on Youtube! Chung (Peter) Chieh (Professor Emeritus, Chemistry @ University of Waterloo). Recognize common ions from various salts, acids, and bases. Public domain. What are $$\ce{[Na+]}$$, $$\ce{[Cl- ]}$$, $$\ce{[Ca^2+]}$$, and $$\ce{[H+]}$$ in a solution containing 0.10 M each of $$\ce{NaCl}$$, $$\ce{CaCl2}$$, and $$\ce{HCl}$$? Therefore, the approximation that s is small compared to 0.10 M was reasonable. Adding a common ion prevents the weak acid or weak base from ionizing as much as it would without the added common ion. The very pure and finely divided precipitate of calcium carbonate that is generated is used in the manufacture of toothpaste. Answer Save. The reaction quotient for PbCl2 is greater than the equilibrium constant because of the added Cl-. & &&= && &&\mathrm{\:0.40\: M} Overall, the solubility of the reaction decreases with the added sodium chloride. Consideration of charge balance or mass balance or both leads to the same conclusion. The lead(II) chloride becomes even less soluble, and the concentration of lead(II) ions in the solution decreases. 1. Adding a common ion to a system at equilibrium affects the equilibrium composition, but not the ionization constant. Consider the lead(II) ion concentration in this saturated solution of PbCl2. The common-ion effect can be used to separate compounds or remove impurities from a mixture. Introduction The solubility products K sp 's are equilibrium constants in hetergeneous equilibria (i.e., between two different phases). Adding the common ion of hydroxide shifts the reaction towards the left to decrease the stress (in accordance with Le Châtelier's Principle), forming more reactants. This page looks at the common ion effect related to solubility products, including a simple calculation. Wikibooks Calculate the molar solubility of a compound in solution containing a common ion. For example, if to a saturated solution of Ag 2 CrO 4 some AgNO 3 has added the solubility of Ag 2 CrO 4 decreases. Solving the equation for s gives s= 1.62×10-2 M. The coefficient on Cl- is 2, so it is assumed that twice as much Cl- is produced as Pb2+, hence the '2s.' The calculations are different from before. If several salts are present in a system, they all ionize in the solution. Different common ions have different effects on the solubility of a solute based on the stoichiometry of the balanced equation. The solubility products Ksp's are equilibrium constants in hetergeneous equilibria (i.e., between two different phases). The way in which the solubility of a salt in a solution is affected by the addition of a common ion is discussed in this subsection. & && && + &&\mathrm{\:0.10\: (due\: to\: HCl)}\\ \begin{alignat}{3} http://en.wiktionary.org/wiki/precipitate, http://en.wikipedia.org/wiki/Common_ion_effect, http://en.wikibooks.org/wiki/Chemical_Principles/Solution_Equilibria:_Acids_and_Bases%23Common-Ion_Effect, http://commons.wikimedia.org/wiki/File:Lithium_hydroxide_with_carbonate_growths.JPG, https://www.boundless.com/chemistry/textbooks/boundless-chemistry-textbook/. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. 9th ed. The following examples show how the concentration of the common ion is calculated. Lead thiocyanate, Pb(SCN)2, has a Ksp of 2.00 x 10^-5. \(\mathrm{[Na^+] = [Ca^{2+}] = [H^+] = 0.10\: \ce M}. Sodium carbonate (chemical formula Na2CO3) is added to the water in order to decrease the hardness of the water. In the water treatment process, sodium carbonate salt is added to precipitate the calcium carbonate. CC BY-SA 3.0. http://en.wikibooks.org/wiki/Chemical_Principles/Solution_Equilibria:_Acids_and_Bases%23Common-Ion_Effect John poured 10.0 mL of 0.10 M $$\ce{NaCl}$$, 10.0 mL of 0.10 M $$\ce{KOH}$$, and 5.0 mL of 0.20 M $$\ce{HCl}$$ solutions together and then he made the total volume to be 100.0 mL. Boundless Learning $$\mathrm{[Cl^-] = \dfrac{0.1\: M\times 10\: mL+0.2\: M\times 5.0\: mL}{100.0\: mL} = 0.020\: M}$$. This is because Le Chatelier’s principle states the reaction will shift toward the left (toward the reactants) to relieve the stress of the excess product. $PbCl_2(s) \rightleftharpoons Pb^{2+}(aq) + 2Cl^-(aq)$. The reaction then shifts right, causing the denominator to increase, decreasing the reaction quotient and pulling towards equilibrium and causing Q to decrease towards K. Adding a common ion decreases solubility, as the reaction shifts toward the left to relieve the stress of the excess product. I need to look again at a simple solubility product calculation, before we go on to the common ion effect. This type of response occurs with any sparingly soluble substance: it is less soluble in a solution which contains any ion which it has in common. The number of ions coming from the lead(II) chloride is going to be tiny compared with the 0.100 M coming from the sodium chloride solution. Scientists take advantage of this property when purifying water. Harwood, William S., F. G. Herring, Jeffry D. Madura, and Ralph H. Petrucci. \end{alignat}\). The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. What is the common ion effect? The 2s term is << 0.10 moles per liter, and therefore: This approximation is also valid, since only 0.0019 percent as much CaF2 will dissolve in 0.10 M NaF as in pure water. $$\mathrm{CaCl_2 \rightleftharpoons Ca^{2+} + {\color{Green} 2 Cl^-}}$$ The common-ion effect is used to describe the effect on an equilibrium involving a substance that adds an ion that is a part of the equilibrium. Contributions from all salts must be included in the calculation of concentration of the common ion. Calculate ion concentrations involving chemical equilibrium. When equilibrium is shifted toward the reactants, the solute precipitates. The balanced reaction is, $PbCl_{2 (s)} \rightleftharpoons Pb^{2+} _{(aq)} + 2Cl^-_{(aq)}$. Finally, compare that value with the simple saturated solution: The concentration of the lead(II) ions has decreased by a factor of about 10. At first, when more hydroxide is added, the quotient is greater than the equilibrium constant. Boundless vets and curates high-quality, openly licensed content from around the Internet. The reaction is put out of balance, or equilibrium. Solution Favorite Answer. 1 Answer. CC BY-SA 3.0. http://en.wiktionary.org/wiki/precipitate $$\mathrm{NaCl \rightleftharpoons Na^+ + {\color{Green} Cl^-}}$$ since fluoride ions are in NaF as well as in CaF2. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org.