Module 4 Intro

Module 4—Properties of Solutions

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Understanding the Solubility Table

 

One of the most important tools you will use while doing chemistry is a solubility table. A solubility table is used to determine the state of an ionic substance when it is placed in water. Copies of this table are also located inside the back cover of your textbook and in the Chemistry Data Book.

 

Ion	H+, Na+, NH4+, NO3–, ClO3–, ClO4–, CH3COO–	F–	Cl–, I–, Br–	SO42–	CO32–, PO43–, SO32–,	IO3–, C2O42–	S2–	OH–
Solubility greater than or equal to 0.1 mol/L (very soluble)	most except: RbClO4, CsClO4, AgCH3COO, Hg2 (CH3COO)2	most	most	most	H+, Na+, K+, NH4+ Except: Li2CO3	H+, Na+, K+, NH4+, Li+, Ni2+, Zn2+ Except: Co(IO3)2, Fe2(C2O4)3	H+, Na+, K+, NH4+, Li+, Mg2+, Ca2+	H+, Na+, K+, NH4+, Li+, Sr2+, Ca2+, Ba2+
Solubility less than 0.1 mol/L (slightly soluble)	none	Li+, Mg2+, Ca2+, Sr2+, Ba2+, Fe2+, Hg22+, Pb2+	Cu+, Ag+, Hg22+, Hg2+, PbI2	Ca2+, Sr2+, Ba2+, Hg22+, Pb2+, Ag+	most	most	most	most

 

You may wonder why the solubility table says “very soluble” and “slightly soluble” instead of simply saying a solution is aqueous or solid. The reason these phrases are used is because all substances dissolve to some extent, so even a relatively insoluble compound, like CaCO3(s), may have a few errant ions in solution. However, for the sake of simplicity, you are allowed to use the aqueous state (aq) for “very soluble” and the solid state (s) for “slightly soluble” because the vast majority of entities will exist in these states.

 

Example 1: Determine the state of calcium carbonate in water.

 

First, locate the carbonate ion in the top row, and then locate the calcium ion underneath. Since the calcium ion is not listed as one of the ions in the "very soluble" section, it must be in the “most” category.

 

Calcium carbonate has low solubility in water, so it can be written CaCO3(s).

 

Example 2: Determine the state of barium hydroxide in water.

 

First, locate the hydroxide ion in the top row, and then locate the barium ion underneath. Barium hydroxide has high solubility in water, so it can be written Ba(OH)2(aq).

 

While the solubility table provides a lot of useful information, it is limited to predicting the state of ionic solids in water. Since you will need to predict the states of many other substances in water, you should memorize the following rules and apply them when you write dissociation equations:

• Elements have low solubility in water. For example, a copper wire in water would be Cu(s) and nitrogen gas bubbled through water is N2(g).
  

• Liquid water is always written H2O(l). It is never written H2O(aq) because water cannot dissolve in itself.
  

• All ions are soluble and written with the state (aq).
  

• Polar molecules will dissolve in water to become aqueous solutions. Nonpolar molecules do not dissolve in water and retain their original state.
  

Now work through the following examples to put these four rules into use.

 

Example 3: Determine the state of ethanol, C2H5OH(l), in water.

 

Ethanol is a polar molecule, so it will dissolve in water to become C2H5OH(aq).

 

Example 4: Determine the state of octane, C8H18(l), in water.

 

Octane is a nonpolar molecule and does not dissolve. It stays as C8H18(l).

 

Example 5: State the major entities present when Sr(OH)2(s) is placed in water.

 

According to the solubility chart, Sr(OH)2(s) is highly soluble, so it will dissociate. The major entities present in water will be Sr2+(aq), OH–(aq), and H2O(l). When you are asked to state the major entities present, coefficients are not required and water should be included in the list since solvent molecules also count.

 

Example 6: Write the balanced dissociation equation for sodium sulfate.

 

According to the solubility chart, Na2SO4(s) is soluble in water, so the balanced dissociation equation is

Na2SO4(s) → 2 Na+(aq) + SO42–(aq).
 
When you are asked to write a dissociation equation, coefficients are required. However, water does not need to be written in a dissociation equation since it is assumed to be present on both sides.