In chapter 4 we defined solubility as the amount of a substance that dissolves in a given quantity of solvent at a given temperature to form a saturated solution. We approached solubility in a very simple manner by classifying salts as soluble or insoluble and solubility guidelines were established indicating if a precipitate would form when two solution are combined.
17.4 Overview of Solubility
Experimentally, we can consider solubility equilibria to make predictions about the amount of a given salt that will dissolve. First, we need to define what we mean by solubility and we need to consider what we already know about solubility and compare that to what a chemist thinks about when they hear the term solubility. When chemists discuss solubility they do so in terms of the solubility product constant. Keep in mind that the solubility product constant and molar solubility are two completely different terms. As chemists, we are able to calculate one from the other, but directly comparing the two without a calculation can lead to some improper assumptions. It is very important to recognize the similarities/differences between solubility in terms of the amount (in grams) that dissolve per liter of solution, the molar solubility, and the Ksp.
17.4 The Solubility Product Constant
Example Problem: 1.4 x 10-6 grams of ZnCO3 dissolve in 1.000 mL of solution and 2.8 x 10-6 grams of BaCrO4 dissolve in 1.000 mL of solution, which one has the largest Ksp? Solution: They are the same. Click below to see the explanation.
One common mistake students make is to simply look at the Ksp and think that constant tells them everything they need to know about the solubility when two slightly soluble salts are compared. If you are thinking this way, then you certainly aren’t thinking like a chemist. A chemist would think through the process shown in the example below.
Ranking the solubility of slightly soluble salts given the Ksp Part I
17.4 Ranking the solubility of slightly soluble salts given the Ksp Part II
Homework:
Read Section 17.4 and the Ksp Lab
Mastering Chemistry Solubility Constant Expression Tutorial
Mastering Chemistry Introduction to Solubility and the Solubility Product Constant Tutorial
Mastering Chemistry The Solubility Product in Medicine Tutorial
*Mastering Chemistry Ksp Pre-Lab Assignment
*Solubility Graded Homework #1
*Solubility Graded Homework #2
Questions 1-18 from the Solubility homework set posted on Carmen.
LAB #1
DEVELOPING A MASTERY OF THE SOLUBILITY PRODUCT CONSTANT LAB
Once you have a handle on saturated aqueous solutions, we can start to analyze and manipulate the solubility of these solutions and determine the criteria for precipitation.
17.6 Criteria For Precipitation
With a background on precipitate formation, we can now predict if a precipitate will form when two solutions are combined.
17.6 If Two Solutions are Mixed Will a Precipitate Form?
In certain cases, it is beneficial to separate a mixture of ions in solution. By utilizing the Ksp values and performing calculations, we can calculate the concentration needed to give the best separation of ions in solution.
17.6 Order of Precipitation, Minimum Concentration Needed to Facilitate Precipitation, and Best Separation
Homework:
Read Section 17.6
Mastering Chemistry Precipitation Tutorial
Mastering Chemistry Selective Precipitation Tutorial
Mastering Chemistry Precipitation Calculation Tutorial
Mastering Chemistry Fractional Precipitation of Metal Carbonates Tutorial
*Solubility Graded Homework #3
Questions 58-69 from the Solubility homework set posted on Carmen.
A common laboratory experiment constructed to illustrate the principles of solubility is Qualitative Analysis. This experiment is designed to answer what is present. Unfortunately, there is no spot test for each individual cation, so a scheme was developed, which groups cations based on their solubility characteristics. Each ion is then isolated and identified.
17.7 Applied Qualitative Analysis Scheme
The Group I cations precipitate as chlorides under acidic conditions.
17.7 Group I Separations
17.7 Why does our HCl need to be cold and dilute?
17.7 Group I Analysis
APPLIED QUALITATIVE ANALYSIS LAB
Similar analysis is performed for the Group II and Group III cations and is investigated in detail in your lab manual.
Chemists love to manipulate things. Ksp expressions are equilibrium expressions and as Le Chatlier showed us, equilibria can be manipulated if you apply a stress. There are many factors that influence solubility and the factors we will investigate in this class are: Common Ion, pH, Complex Ion Formation, and Amphoterism. In all of these effects, it is very important to WRITE OUT THE EQUILIBRIUM EXPRESSION so you can properly analyze how the solubility will be influenced.
The Common Ion effect is simply a direct application of Le Chatlier’s principle.
17.5 Common Ion Effect
Homework:
Mastering Chemistry Solubility of “Insoluble” Salts Tutorial
Mastering Chemistry The Common-Ion Effect Tutorial
Mastering Chemistry Common-Ion Effect on Solubility for Lead Thiocyanate Tutorial
Mastering Chemistry Common-Ion Effect on Solubility for a Metal Hydroxide Tutorial
*Solubility Graded Homework #4-#5
The pH of a solution has a dramatic influence of the solubility. When investigating pH effects, it is important for you to know the strong acids, which will allow you to identify all the neutral anions in solution.
17.5 pH Effects
17.5 How does adding acid/base influence solubility?
Homework:
Mastering Chemistry The Effect of Acid on Solubility Tutorial
Mastering Chemistry Effect of pH on Solubility Tutorial
Mastering Chemistry Acid Rain: Effect on Solubility of Calcium Carbonate Tutorial
*Solubility Graded Homework #6-#7
Mastering Chemistry Creating a Buffer Solution Tutorial
Mastering Chemistry pH of a Buffer Solution Tutorial
*Solubility Graded Homework #8-#9
Questions 24-32 from the Solubility homework set posted on Carmen
Le Chatlier’s principle does a great job of explaining the solubility in terms of the common ion effect and pH effects, but in some instances some puzzling results are obtained. For instance, when concentrated NH3 is added to a saturated solution of Zn(OH)2 its solubility increases. Now we need to determine why this is, and if Le Chatlier’s principle is still valid.
17.5 Does zinc hydroxide follow the rules we’ve discussed so far?
It turns out Le Chatlier’s principle still applies, but there is something else going on in solution responsible for these results. The concept responsible for the difference in solubility is complex ion formation.
17.5 Complex ion formation and coordination complexes
This allows us to investigate zinc hydroxide in a more complex way and it allows us to perform several calculations based on complex ion formation.
17.5 Re-analyzing zinc hydroxide
17.5 Solubility of zinc hydroxide in 15 M NH3
17.5 Determining the concentration of free metal cations in solution
Amphoterism is a special case where a slightly soluble salt has its solubility increased due to pH effects in acidic solution and has its solubility increased due to complex ion formation under basic conditions.
17.5 Amphoterism
17.5 Solubility of Al(OH)3 in 15 M NH3
17.5 Molar Solubility of Al(OH)3 in 15 M NH3 continued
17.5 Amphoteric Effects on Solubility
Homework:
Mastering Chemistry Solubility of Zinc Hydroxide in Basic Solution
Mastering Chemistry Cyanide Poisoning
*Solubility Graded Homework #10-#12
Questions 33-46 from the Solubility homework set posted on Carmen
The theory behind the separations of the Group II and Group III encompass all these effects and utilizes the solubility of the sulfide ion in solution.
17.7 Group II and Group III Sulfide Solubility
17.7 Will FeS precipitate?
17.7 At what pH will FeS begin to precipitate?
Homework:
Read Section 17.7 and the Qualitative Analysis Lab
Mastering Chemistry Qualitative Analysis of Metal Cations Tutorial
Mastering Chemistry Selective Precipitation in an Acidic Solution Tutorial
*Solubility Graded Homework #13-#14
Questions 19-23 & 72-84 from the Solubility homework set posted on Carmen
Keeping the following in for the footnotes
When chemists discuss solubility they do so in terms of the solubility product constant. Keep in mind that the solubility product constant and molar solubility are two completely different terms. 11. Well, this is an example of what we can do with footnotes. As chemists, we are able to calculate one from the other, but directly comparing the two without a calculation can lead to some improper assumptions. It is very important to recognize the similarities/differences between solubility in terms of the amount (in grams) that dissolve per liter of solution, the molar solubility, and the Ksp.
One common mistake students make is to simply look at the Ksp and think that constant tells them everything they need to know about the solubility when two slightly soluble salts are compared. If you are thinking this 33. I really am terrified to see what this looks like in Internet Explorer. Microsoft = Ugh.way, then you certainly aren’t thinking like a chemist. A chemist would think through the process shown in the example below.
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