Today in lecture I started on our first "unit," which is SOLUBILITY. Solubility was first introduced to us back in Chapter 4 of the textbook and we were only given a chart of "solubility guidelines" telling us if an ionic substance is "soluble" or "insoluble" in aqueous solution. This table is a way to represent solubility in a qualitative sense, but in this course we will look at solubility in a quantitative sense, or we will determine the exact amount of a substance that dissolves.
To determine just how much solute will dissolve in an aqueous solution, chemists can analyze the equilibrium constant, or Ksp. The Ksp is a measure of how much of a solid dissolves to form a saturated solution.
Typically Ksp tables are generated for various inorganic compounds. The chart listed here is shown in the appendix of the lab manual. There is a relationship between these two tables. Focus a minute on the chlorides in the solubility rules table. All chlorides are soluble with the exception of Ag+, Pb2+, and Hg22+ . These are all insoluble, or will form precipitates. All other chlorides will remain in solution. This can now be used as an effective separation technique if we want to isolate Ag+, Pb2+, and Hg22+ from any other cations. This is exactly the case for Exp #25 Group I in the qualitative analysis. If you look at the following figure you can now start to see the relationship between these two tables. If a compound is "insoluble," or more accurately, "slightly soluble" there will be a Ksp value associated with it. The smaller the value for the Ksp, the less soluble a substance is. Let me reiterate that The Ksp is a measure of how much of a solid dissolves to form a saturated solution.
I also discussed Molar Solubility and started class giving you a list of five compounds and asked which one had the largest molar solubility. This is a very common question you will encounter on my quizzes/exams and it will definitely show up on standardized exams such as the MCAT.
Remember that the solubility of a substance is the quantity that dissolves to form a saturated solution, while the the Ksp is the equilibrium constant established and its magnitude can be used to make predictions about solubility. We can directly relate the solubility of compounds with the same ratio of cation:anion, but if two compounds have a different ratio of cation:anion you must perform a calculation using an ICE table in order to predict which compound is more (or less) soluble.
Chemists love the fact that they can predict what will happen in the lab before it even happens. The question we want to ask ourselves now is how can we predict whether a precipitate forms when we add two solutions together? In chemistry 121 the solubility rules were our guide for this, but these guidelines were not very specific about what concentration must be used to facilitate a precipitation reaction. We can selectively separate when various precipitates form by manipulating the concentrations of ions that are mixed together. The reaction quotient, Q, is obtained when concentrations of reactants are inserted into an equilibrium expression. If we are at equilibrium we have a special case of Q that we refer to as K. I performed an example of Q vs. K by analyzing solutions of silver nitrate and potassium chloride.
If you are asked to predict if a precipitate will form you need to calculate Q and compare it to K. In general, if:
Q < K no precipitate forms
Q = K the solution is at equilibrium
Q > K a precipitate will form
We will use these same concepts of solubility to discuss the Qualitative Analysis scheme in Exp. #25 in class on Friday.
I will try my best to keep this blog updated after every lecture and I will post the materials you will be responsible for on the quizzes and exams on here as well. As always I would appreciate any comments or suggestions as to how to make this resource more valuable to you as a student.
To be keeping up with the material up to this point you should do the following:
*Go to Carmen and read the entire syllabus and know what your expectations and responsibilities are for this course.
*Read Section 17.4, 17.6, and 17.7 in the textbook.
*Read pages 35-37 and 39-45 of the lab manual and start working on the pre-lab for Exp #25 Group I.
*Complete problems 1-18 on the solubility homework set.
Song Played Before Class:
The Who: Baba O'Riley
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