Some relevant definitions:

Electrolyte:  This is a compound which forms hydrated ions in solution.  It resultant solution has the property that it conducts electricity to some extent.

Non electrolyte:  This is a compound which does not form hydrated ions in solution.  It resultant solution has the property that it conducts electricity about as poorly as pure water..

Strong electrolyte:  This electrolyte completely (>98%) dissociates into the hydrated ions in solution.  It resultant solution has the property that it conducts electricity very well.

Weak electrolyte:  This electrolyte dissociates into the hydrated ions in solution to only a limited extent.  It resultant solution has the property that it conducts electricity poorly, but better than pure water.

What is the van't Hoff factor:^footnote1

The van't Hoff factor, symbol i,  expresses how may ions and particles are formed (on an average) in a solution from one formula unit of solute.

    Examples:

• One formula unit of NaCl will create two particles in solution, a Na⁺  ion and a Cl^- ion.
• One formula unit of CaCl₂ will create three particles in solution, a Ca⁺  ion and two Cl^- ions.
• One formula unit (molecule) of methanol CH₃OH will create only one particle in solution, since methanol is a non electrolyte and does not dissolve as ions.
• One formula unit (molecule) of HF will form one particle in solution, since it is a weak electrolyte (weak acid).^footnote2
• One formula unit (molecule) of NH₃ will form one particle in solution, since it is a weak electrolyte (weak base).^footnote2

The first step is to be able to recognize whether one is dealing with an electrolyte or a non electrolyte.  It is easier to recognize electrolytes, although the student by this point should be able to recognize the following organic functional groups which are associated with non electrolytes (not a complete list).  The dashes, - and >, correspond to covalent bonds to another atom, usually C.

• The alcohol group -OH
• The carboxylic acid group -COOH
• The amine groups:   primary amine  -NH₂,  secondary amine >NH and ternary amine >N-
• The ether group   -O-

To recognize electrolytes:

All acids, bases and salts are electrolytes.  The definitions of these terms are found in CHE 121 handouts.  This includes the salts and acids of the polyions.  Thus, you need to know your polyion list which is in your lab manual or in the CHE 121 handouts.  You must also know this list in order!  Below is the some of the list in the form of the acids.  Notice the lines which delineate the 8 acids in the upper right.
 

Strong acids: (belong to the class of strong electrolytes)

The strong acids completely break up into hydronium ions and the anions in solution.  The following are the strong acids:

The binary acids HCl, HBr and HI  (the hydrohalic acids except for HF).  Thus, for example, HCl dissolves in water by the reaction:

                HCl (g)  +  H₂O   -->   H₃O⁺   +   Cl^-

This gives a van't Hoff factor or 2.   (i = 2)

The 8 ternary acids in the upper right side of the table above (to the right and above the blank non-tabled boxes presented) are all strong^footnote3.  Thus, for example, HClO4 dissolves in water by the reaction:

                HClO₄   +  H₂O   -->   H₃O⁺   +   ClO₄^-

This gives a van't Hoff factor or 2.   (i = 2)

All other acids are weak, including the organic acids which have the functional group -COOH.

Weak Acids:   (belong to the class of weak electrolytes)

All weak acids have a van't Hoff factor of approximately 1.

Example:  The acid HClO₂ has a van't Hoff factor of 1.  This acid is, according to the table above, a weak acid.

Example:  The acid HF has a van't Hoff factor of 1.  This binary acid is not HCl, HBr or HI and is therefore a weak acid.

Example:  The acid CH₃COOH has a van't Hoff factor of 1.  This acid is not a mineral acid, but rather an organic acid.  All organic acids, if they dissolve, are weak acids in water solutions.

Strong Soluble Bases:

The strong soluble bases are the hydroxide compounds of group I metals and group II metals from Ca down on the periodic chart.  All other metal hydroxides are insoluble bases and only a very small amount will dissolve in water.  (Notice that OH^- was on your polyion list!)

Examples:

LiOH has a van't Hoff factor of two.   LiOH is a strong soluble base (group 1 metal hydroxide) and dissolves in water according to:

    LiOH (s)  -->  Li⁺  +  OH^-

Thus, each formula unit yields two ions total.

Ca(OH)₂ has a van't Hoff factor of three.   Ca(OH)₂ is a strong soluble base (group 1 metal hydroxide) and dissolves in water according to:

    Ca(OH)₂ (s)  -->  Ca²⁺  +  2OH^-

Thus, each formula unit yields three ions total.

Insoluble Bases:

These are irrelevant since they are insoluble in water.  Examples include:  Mg(OH)₂,  Fe(OH)₃,
Zn(OH)₂, etc.

Weak Soluble Bases:

Most weak soluble bases may be recognized by being nitrogen compounds.  These are either amines (see above) or are nitrogen compounds with nitrogen covalently bonded to hydrogen.

Examples:

    ammonia  NH₃
    methyl amine  CH₃NH₂
    hydrazine  H₂NNH₂
 

 Salts are Strong Electrolytes:^footnote4

 Salt break up into their ions.  For binary salts break into the individual metal ions (cations) and nonmetal ions (anions)

Some typical examples:

NaCl(s)  -->  Na+  +  Cl-	i = 2
Li2S(s)  -->  2Li+  +  S2-	i = 3
CaBr2(s)  -->  Ca2+  +  2Br-	i = 3
FeCl3(s)  -->  Fe3+  +  3Cl-	i = 4

For salts with polyions (here's where you need to know your polyions - learn them if you haven't by now) break up into metal ion (cation) and the polyion as a unit.

Some typical examples

Na2SO42- (s)  -->  2Na+  +  SO42-	i = 3
Fe3(PO4)2 (s)  -->  3Fe2+  +  2 PO43-	i = 5

Summary:

All water soluble compounds have a van't Hoff factor of approximately 1 except:

    Compounds which begin with a metal ions.  These include salts and strong bases
    Strong acids - so you need to be able to recognize a strong acid.
    Strong soluble bases - so you need to be able to recognize a strong soluble base.     Back to top

footnote 2:  Both weak acids and weak bases dissociate to a small extent, but not enough to change the conclusion about the approximate van't Hoff factor.  This dissociation will become a topic of considerable study in the next few sections of CHE 122 BACK

footnote 3:  H₂SO₄ is a strong acid for only the removal of the first proton.  HSO₄^- is a weak acid.  This complication will be avoided for the present, but will be revisited with the lead acid battery.    BACK

footnote 4:  Ok, Ok!  I know there are some very rare exceptions, after all I've worked with water glass.  We will avoid the exceptions.  If the student runs into some in the future, let the amazement flow.     BACK