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Solubility Product Constant (Ksp)

Introduces and defines solubility product constant (Ksp).

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What are the Chances?

How do the chances of a substance dissolving change with the solubility product constant?

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The lottery -- we often see stories about it on the news, of how the Powerball Lottery reached its highest all-time jackpot, or of how lucky winners made millions overnight. Often, when these exciting events occur, probability analysts will come on news talk shows to discuss the chances of winning certain lotteries. Maybe the chances for the Texas Lottery are 1 in 856,879; or 1 in 175,711,536 for the US Mega Millions jackpot! Whatever the statistical chances, your chances of winning increase as the denominator decreases, and vice versa.

The solubility product constant, or Ksp, works in a similar way. Your “analyst,” whether it is a textbook, the Internet, or the background information for a question, provides you with the Ksp for certain substances. This Ksp gives you the ratio of ions to the substance they comprise. It also tells you how readily the substance will dissolve in water. The larger the Ksp, the more readily it dissolves, and vice versa.

Creative Applications

  1. Given two reactions, each with its own precipitate and provided Ksp, how might you determine which one precipitates first?
  2. Calcium sulfate has a Ksp of about 7.1 × 10-5. Compared to barium chromate (Ksp = 1.2 × 10-10), how soluble is it in water?

  3. Research: Look for another common solvent aside from water and try to determine how it affects solubility of substances. You may use the link provided in the "Resources" section. (Hint: think about the solubility in terms of polarity.)




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