A **polyprotic acid** possesses more than 2 hydronium ions (H^{+}).

Our understanding of diprotic acids and bases can be used to understand polyprotic acids and bases.

**Concept:** The most common type of polyprotic acid is the triprotic acid, which contains 3 acidic hydronium ions.

**Concept:** Triprotic bases contain 3 K_{b} values, so they would have 3 equilibrium equations.

**Concept:** As a result of these equations for polyprotic acids and bases the relationship between K_{a} and K_{b} can be established.

**Example:** Determine the pH of 0.300 M sodium hydrogen phosphate, Na_{2}HPO_{4}. Phosphoric acid, H_{3}PO_{4}, contains K_{a1} = 7.5 x 10** ^{-3}**, K

**Example:** Determine the pH of 0.300 M citric acid, H_{3}C_{6}H_{5}O_{7} it possesses K_{a1} = 7.4 x 10** ^{-4}**, K

Phosphorous acid dissociates to form dihydrogen phosphate and hydronium ion. Phosphorous acid has K_{a} of 5.0 x 10^{-}^{2}.

Write the equation dissociation for phosphorous acid. Express your answer as a chemical equation. Identify all of the phases in your answer.

What is the expression constant dissociation for phosphorous acid?

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Phosphoric acid is a triprotic acid (Ka1 = 6.9 x 10^-3, Ka2 = 6.2x 10^-8, and Ka3 = 4.8 x10^-13). To find the pH of a buffer composed of H2PO4^- (aq) and HPO4^2- (aq), which pKa value would you use in the Henderson-Hasselbalch equation?

Calculate the pH of a buffer solution obtained by dissolving 13.0 g of KH2PO4(s) and 26.0 g of Na2HPO4(S) in water and then diluting to 1.00 L

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Cysteine is an amino acid with R=-CH_{2}SH, which ionizes according to:

The three P*k _{a}*'s of cysteine are: p

If one prepares 0.40 M solution of cysteine and the pH is adjusted to pH=8.75, calculate the molar concentrations, [Cys] and [Cys-], in the resulting solution.

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Identify the triprotic acid.

A) HNO_{3}

B) H_{3}PO_{4}

C) H_{2}CO_{3}

D) HFO_{4}

E) H_{2}SO_{4}

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Farmers who raise cotton once used arsenic acid, H_{3}AsO_{4} , as a defoliant at harvest time. Arsenic acid is a polyprotic acid with K_{1} = 2.5 x 10^{-4} , K _{2} = 5.6 x 10^{-8} , and K_{3} = 3 x 10^{-13} . What is the pH of a 0.500 M solution of arsenic acid?

A) 0.85

B) 1.95

C) 3.90

D) 4.51

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Consider a solution that is 0.10 M in a weak triprotic acid which is represented by the general formula H_{3}A with ionization constants *K*_{1 }= 1.0 × 10^{−3} , *K*_{2} = 1.0 × 10^{−8} , and K_{3} = 1.0 × 10^{−12}. What is the pH of the solution?

1. 2.15

2. 2.25

3. 2.02

4. 2.35

5. 2.54

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Determine the pH of a 0.03 M solution of NaH _{2}PO_{4}? Assume H_{3}PO_{4} has a p*K*_{a1} of 2.1 and a p*K*_{a2} of 7.2 and a p*K*_{a3} of 12.7.

1. 4.36

2. 7.11

3. 4.65

4. 7.40

5. 9.95

6. 1.81

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Identify the weak triprotic acid.

A) CH_{3}COOH

B) H_{3}PO_{3}

C) H_{3}PO_{4}

D) H_{2}SO_{4}

E) H_{2}SO_{3}

Watch Solution

For the polyprotic acid H_{3}PO_{4} (phosphoric acid), which of the following expressions would correspond to *K*_{a3}?

1. H_{3}PO_{4} ⇌ PO_{4}^{3−} + 3 H^{+}

2. PO_{4}^{3−} + 3 H^{+} ⇌ H_{3}PO_{4}

3. HPO_{4}^{2−} ⇌ PO_{4}^{3−} + H^{+}

4. H_{2}PO_{4}^{−} ⇌ HPO_{4}^{2−} + H^{+}

5. H_{3}PO_{4} ⇌ H_{2}PO_{4}^{−} + H^{+}

Watch Solution

Phosphoric acid (H_{3}PO_{4}) is characterized by three reactions for stepwise dissociation. Arrange the following species in the order of decreasing concentration in an aqueous solution of H_{3}PO_{4}. Clearly indicate the order of concentration change. JUSTIFY your answer.

H_{3}PO_{4} H _{2}PO_{4}^{-} HPO _{4}^{2-} PO _{4}^{3-} H _{3}O^{+}

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