If you like us, please share us on social media.
The latest UCD Hyperlibrary newsletter is now complete, check it out.

GeoWiki.png
ChemWiki: The Dynamic Chemistry E-textbook > Physical Chemistry > Equilibria > Solubility > Solubility Product Constant, Ksp

MindTouch
Copyright (c) 2006-2014 MindTouch Inc.
http://mindtouch.com

This file and accompanying files are licensed under the MindTouch Master Subscription Agreement (MSA).

At any time, you shall not, directly or indirectly: (i) sublicense, resell, rent, lease, distribute, market, commercialize or otherwise transfer rights or usage to: (a) the Software, (b) any modified version or derivative work of the Software created by you or for you, or (c) MindTouch Open Source (which includes all non-supported versions of MindTouch-developed software), for any purpose including timesharing or service bureau purposes; (ii) remove or alter any copyright, trademark or proprietary notice in the Software; (iii) transfer, use or export the Software in violation of any applicable laws or regulations of any government or governmental agency; (iv) use or run on any of your hardware, or have deployed for use, any production version of MindTouch Open Source; (v) use any of the Support Services, Error corrections, Updates or Upgrades, for the MindTouch Open Source software or for any Server for which Support Services are not then purchased as provided hereunder; or (vi) reverse engineer, decompile or modify any encrypted or encoded portion of the Software.

A complete copy of the MSA is available at http://www.mindtouch.com/msa

Solubility Product Constant, Ksp

The solubility product constant, \(K_{sp}\)​, is the equilibrium constant for a solid substance dissolving in an aqueous solution. It represents the level at which a solute dissolves in solution. The more soluble a substance is, the higher the \(K_{sp}\) value it has.

Consider the general dissolution reaction below:

\[ aA (s) \rightleftharpoons cC (aq) + dD (aq) \]

To solve for the \(K_{sp}\) it is necessary to take the molarities or concentrations of the products (cC and dD) and multiply them. If there are coefficients in front of any of the products, it is necessary to raise the product to that coefficient power(and also multiply the concentration by that coefficient). This is shown below:

\[ K_{sp} = [C]^c [D]^d \]

Note that the reactant, aA, is not included in the \(K_{sp}\) equation. Solids are not included when calculating equilibrium constant expressions, because their concentrations do not change the expression; any change in their concentrations are insignificant, and therefore omitted.

Hence, \(K_{sp}\) represents the maximum amount of solid that can be dissolved in the aqueous solution. The answer will have the units of molarity, mol L-1, a measure of concentration.

Important effects

  • For highly soluble ionic compounds the ionic activities must be found instead of the concentrations that are found in slightly soluble solutions.
  • Common Ion Effect: The solubility of the reaction is reduced by the common ion. For a given equilibrium, a reaction with a common ion present has a lower  \(K_{sp}\)​, and the reaction without the ion has a greater \(K_{sp}\).
  • Salt Effect (diverse ion effect): Having an opposing effect on the \(K_{sp}\) value compared to the common ion effect, uncommon ions increase the \(K_{sp}\) value. Uncommon ions are ions other than those involved in equilibrium.
  • Ion Pairs: With an ionic pair (a cation and an anion), the \(K_{sp}\) value calculated is less than the experimental value due to ions involved in pairing. To reach the calculated \(K_{sp}\) value, more solute must be added.

Example problems:

  • What is the solubility product constant expression for \(MgF_2\)?

MgF2(s) ↔ Mg2+(aq) + 2F-(aq) so the \(K_{sp}\) = [Mg2+][F-]2

  • What is the solubility product constant expression for \(Ag_2CrO_4\)?

Ag2CrO4(s) ↔ 2Ag+(aq) + CrO42-(aq) so the \(K_{sp}\) = [Ag]+2[CrO42-]

References

  1. Petrucci, Ralph H., et al. General Chemistry: Principles and Modern Applications. Upper Saddle River, NJ: Prentice Hall 2007.

Contributors

  • Kathryn Rashe, Lisa Peterson

You must to post a comment.
Last Modified
08:31, 20 Oct 2014

Tags

Classifications

(not set)
(not set)

Creative Commons License Unless otherwise noted, content in the UC Davis ChemWiki is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License. Permissions beyond the scope of this license may be available at copyright@ucdavis.edu. Questions and concerns can be directed toward Prof. Delmar Larsen (dlarsen@ucdavis.edu), Founder and Director. Terms of Use