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pH Calculations of Weak Acids

 

Basic Information

1) Weak acids are less than 100% ionized in solution. They have a small Ka value.
2) Acetic acid (formula = HC2H3O2) is the most common weak acid example used by instructors.
3) Another way to write acetic acid's formula is CH3COOH.
4) A common abbreviation for acetic acid is HAc, where Ac¯ refers to the acetate polyatomic ion.

 

Ka of Weak Acids

Name Formula Ka

acetic

HC2H3O2

1.8 x 10-5

ascorbic (I)

H2C6H6O6

7.9 x 10-5

ascorbic (II)

HC6H6O6-

1.6 x 10-12

benzoic

HC7H5O2

6.4 x 10-5

boric (I)

H3BO3

5.4 x 10-10

boric (II)

H2BO3-

1.8 x 10-13

boric (III)

HBO32-

1.6 x 10-14

carbonic (I)

H2CO3

4.5 x 10-7

carbonic (II)

HCO3-

4.7 x 10-11

citric (I)

H3C6H5O7

3.2 x 10-7

citric (II)

H2C6H5O7-

1.7 x 105

citric (III)

HC6H5O72-

4.1 x 10-7

formic

HCHO2

1.8 x 10-4

hydrazidic

HN3

1.9 x 10-5

hydrocyanic

HCN

6.2 x 10-10

hydrofluoric

HF

6.3 x 10-4

hydrogen peroxide

H2O2

2.4 x 10-12

hydrogen sulfate ion

HSO4-

1.2 x 10-2

hypochlorous

HOCl

3.5 x 10-8

lactic

HC3H5O3

8.3 x 10-4

nitrous

HNO2

4.0 x 10-4

oxalic (I)

H2C2O4

5.8 x 10-2

oxalic (II)

HC2O4-

6.5 x 10-5

phenol

HOC6H5

1.6 x 10-10

propanic

HC3H5O2

1.3 x 10-5

sulfurous (I)

H2SO3

1.4 x 10-2

sulfurous (II)

HSO3-

6.3 x 10-8

uric

HC5H3N4O3

1.3 x 10-4

 

  

 

An other weak acid example

Vinegar is a dilute water solution of acetic acid with small amounts of other components. Calculate the pH of bottled vinegar that is 0.667 M HC2H3O2, assuming that none of the other components affect the acidity of the solution.

HC2H3O2(aq)      H+(aq) + C2H3O2-(aq)

We get the value for the acid dissociation constant for this reaction from the table above.

**x in the denominator, is considered too small compared to 0.667, so it is ignored.

x2 = 1.2 x 10-5 x = 3.5 x 10-3

[H+] = 3.5 x 10-3 M H+       pH = -log(3.5 x 10-3) = 2.46

 

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