Weak Acids

Weak Acids – dissociate incompletely (~20%)               

Strong Acids – dissociate completely (~100%)

A_(g)  +  2 B_(g)                 3 C_(g)  +  D_(g)

Equilibrium constant (K_eq)    =                        K_eq =      LeChatelier’s

                                                                                                                            (lu-SHAT-el-YAY’s)

Example Problems:  Pg 521 (black book) & pg 519 (transparency)

IONIZATION  CONSTANTS  for  ACIDS

  HC₂H₃O_{2( aq)}              H⁺_(aq)  +  C₂H₃O₂^1-_(aq)

 (Please note, in all examples I incorrectly typed C₂H₅O₂^1- instead of C₂H₃O₂^1-)

Equilibrium constant       K_eq =   =  K_a  =  Acid dissociation constant

K_a  =  1.8 x 10^-5  @ 25 ^oC

K_a  = 

 1.8 x 10^-5   = 

                                           K_a

            HCl             H⁺  +  Cl^1-                             very large

        HNO₃              H⁺  +  NO₃^1-                        very large

        H₂SO₄             H⁺  +  HSO₄^1-                        large

        HC₂H₃O₂                H⁺  +  C₂H₃O₂^1-         1.8 x 10^-5  

        H₂S                  H⁺  +  HS^1-                         9.5 x 10^-8   

_{Sample 1)} 

One gram of pure sulfuric acid (H₂SO₄) is diluted to a 1.0 dm³ volume with water.  What is the molar concentration of the hydrogen ion in this solution? 

What is the pH?

_Solution) 

First determine the number of moles of H₂SO₄

x mol H₂SO₄  = 1 g H₂SO₄   =  0.010 mol H₂SO₄

H₂SO₄              H⁺  +  HSO₄^{1-       &}          HSO₄^1-              H⁺  +  SO₄^2-

OVERALL:   

H₂SO₄              2 H⁺  +  SO₄^2-       in dilute solutions...occurs ~100%

        0.010 M                0.020 M       

A volume of 5.71 cm³ of pure acetic acid, HC₂H₃O₂, is diluted with water at 25 ^oC to form a solution with a volume of 1.0 dm³.  What is the molar concentration of the hydrogen ion, H⁺, in this solution?

(The density of pure acetic acid is 1.05 g/cm³.)

Step 1)  Find the mass of the acid

                   Mass of acid  =  density of acid  x  volume of acid

                                          =  1.05 g/cm³   x 5.71 cm³

                                          =  6.00 g

Step 2)  Find the number of moles of acid.  (From the formula of acetic acid, you can calculate that the molar mass of acetic acid is 60 g / mol).

x mol acetic acid  =  6.00 g acetic acid =  0.10 mol acetic acid (in 1 L)

Molarity:  M  =  mol / L       Substitute into equation        M  =  0.10 mol / 1 L

                                                                        M  =  0.1 molar

Step 3)  Find the [H⁺]

          K_a  =     Substitute into equation:     1.8 x 10^-5  = 

                   1.8 x 10^-5  = 

            x²  =  1.8 x 10^-6

x  =  1.3 x 10^-3 molar

H⁺ Concentrations

Note:  although the sulfuric acid is 10x less concentrated than the acetic acid...

it produces > 10x more H⁺

pH  =  - log[H⁺]

Practice Problems:  on a separate sheet of paper, work out the following problems.  Show set-up and work for full credit.

1a)  What is the molar hydrogen ion concentration in a 2.00 dm³ solution of hydrogen chloride in which 3.65 g of HCl is dissolved? 

1b)  pH

2a)  What is the molar concentration of hydrogen ions in a solution containing 3.20 g of HNO₃ in 250 cm³ of solution?

2b)  pH

3a)  An acetic acid solution is 0.25 M.  What is its molar concentration of hydrogen ions?

3b)  pH

4)  A solution of acetic acid contains 12.0 g of HC₂H₃O₂ in 500 cm³ of solution.  What is the molar concentration of hydrogen ions?

ANSWERS:

1a)  0.0500 M             2a)  0.203 M                            

1b)  pH  =  1.3            2b)  pH  =  0.7

3a)  2.1 x 10^-3 M         3b)  pH  =  2.7

4)  2.7 x 10^-3 M