- Define a polyprotic acid.
- Explain how a polyprotic behaves in its solution.
- Write the equilibrium equations of ionization of polyprotic acids.
- Calculate acidity constants,
*K*_{1},*K*_{2},*K*_{3}, and the overall*K*. - Calculate the concentrations of various species for a given set of data Proof reading and polish still required!

Some Polyprotic Acids | |
---|---|

Formula | Name |

H_{2}S | Hydrogen sulfide |

H_{2}SO_{4} | Sulfuric acid |

H_{2}SO_{3} | Sulfurous acid |

H_{3}PO_{4} | Phosphoric acid |

H_{2}C_{2}O_{4} | Oxalic acid |

H_{2}CO_{3} | Carbonic acid |

H_{2}C_{3}H_{2}O_{4} | Malonic acid |

**Polyprotic acids** contain more than one mole ionizable hydronium ions
per mole of acids. They ionize to give more than one H^{+} ions
per *molecule*. Possible forms of three polyprotic acids are given
below after their dissociation into H^{+} ions.

HThese acids ionize in several stages, giving out one proton at each stage. The acidity constants for these acids may be written as_{2}S, HS^{-}, S^{2-}H_{2}SO_{4}, HSO_{4}^{-}, SO_{4}^{2-}H_{3}PO_{4}, H_{2}PO_{4}^{-}, HPO_{4}^{2-}, PO_{4}^{3-}

Consider H_{2}S,

Hand_{2}S = H^{+}+ HS^{-}[H^{+}] [HS^{-}]K_{1}= ---------- [H_{2}S]

HS^{-}= H^{+}+ S^{2-}[H^{+}] [S^{2-}]K_{2}= ----------- [HS^{-}]

HConfirm the above_{2}S = 2 H^{+}+ S^{2-}[H^{+}]^{2}[S^{2-}]K_{overall}= ----------- [H_{2}S] =K_{1}K_{2}

For polyprotic acids, the following is always true:

**
Examples 1
**

The calculation is straight forward:

= 3.0E-6

**
Example 2
**

*Solution*

[HThus,^{+}]^{2}[S^{2-}]K_{overall}= ----------- [H_{2}S]

0.1 [S^{2-}] = 1E-20 ----- (0.1)^{2}= 1E-19 F

**
Example 3
**

*Solution*

The salt is completely ionized in its solution.

0.100 -

[H

= {-0.120 + (0.012

= 0.0292 M

*Discussion*

Is the NaHSO_{4} salt solution acidic?

Although no concentration is stated, such a solution is acidic because
of the acidity of HSO_{4}^{-}.

**For H _{2}SO_{4}, pKa2 = 1.92**
Work out the answer please, some of these will appear on the examinations.
If the concentration of a salt solution is given, you may be required to
evaluate the pH or pOH of the solution.

For H_{3}SO_{4}, pKa1 = 2.12; pKa2 = 7.21; pKa3 = 12.67

Which of the following solutions are acidic, basic, or neutral?

Na_{2}SO_{4}

NaH_{2}PO_{4}

Na_{2}HPO_{4}

Na_{3}PO_{4}

NaNO_{3}

**
Example 4
**

*Solution*

The 0.500 M solution of NaHSO_{4} supplies 0.500 M HSO_{4}^{-}
as an acid, and similarly, the solution also contains 0.300 M
SO_{4}^{2-}.

Using the equation:

= 1.92 + log (0.300/0.500)

= 1.70

**The two acid ionization constants for sulfurous acid are 1.2E-2 and 6.6E-8 respectively. Calculate the overall equilibrium constant for**H _{2}SO_{3}= 2 H^{+}+ SO_{3}^{2-}*Answer**7.9E-10*

**Consider...***K*_{overall}=*K*_{1}**K*_{2}**What is the pH of a 1.0 M H**_{2}SO_{3}solution?

(*K*_{1}= 1.2E-2, and*K*_{2}= 6.6E-8)*Answer 0.98*Only

**Consider...***K*_{1}matters in this calculation. Using the quadratic formula yields a pH of 0.98. When approximation is used, you'll get a pH of 0.96**If the pH of a 1.0 M H**_{2}SO_{3}solution is 1.0, what is the sulfite ion concentration? (*K*_{1}= 1.2E-2, and*K*_{2}= 6.6E-8)*Answer**7.1E-8*

**Consider...***K*_{overall}= 7.9E-10 = {[H^{+}]^{2}[SO_{3}^{2-}]} / [H_{2}SO_{3}] = 0.1^{2}[SO_{3}^{2-}]/0.9 [SO_{3}^{2-}] = ??