Diprotic Acids

A diprotic acid is an acid that can donate two hydronium ions (H+). 

Diprotic Acids

Since a diprotic acid has two acidic hydrogens it would have 2 equilibrium equations. 

Concept: Diprotic Acids Concept 1

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The diprotic bases would also have 2 equilibrium equations. 

Concept: Diprotic Acids Concept 2

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Concept: As a result of these equations for diprotic acids and bases the relationship between Ka and Kb can be established. 

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Concept: Sulfurous acid, H2SO3, represents a diprotic acid with a Ka1 = 1.6 x 10-2 and Ka2 = 4.6 x 10-5. Calculate the pH and concentrations of H2SO3, HSO3 and SO32– when given 0.250 M H2SO3

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Concept: Determine the pH of 0.115 M Na2S. Hydrosulfuric acid, H2S, contains Ka1 = 1.0 x 10-7 and Ka2 = 9.1 x 10-8.

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Diprotic Acids Additional Practice Problems

What is the pH of a 0.310 M solution of H2SO4? Ka2 = 1.20 x 10^-2


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Write chemical equations and corresponding equilibrium expressions for each of the two ionization steps of carbonic acid.

Express as chemical equation; identify all phases.

a. Write chemical equations for first ionization step of carbonic acid.

b. Write chemical equations for second ionization step of carbonic acid.

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For the diprotic weak acid H2A, Ka1 = 3.1 X 10-6 and Ka2 = 5.1 X 10-9. What is the pH of a 0.0500 M solution of H2A? What are the equilibrium concentrations of H2A and A2- in this solution?

pH = ?

H2A = ?

A2- = ?

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Calculate the equilibrium concentration of C2O42− in a 0.20 M solution of oxalic acid.

Express your answer to two significant figures and include the appropriate units.

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The pKa's for carbonic acid and bicarbonate at 37°C are 3.83 and 10.25, respectively. Write the equation for each of these equilibria.

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H2CO3 is a diprotic acid. The pKa for H2CO3 is 6.35 and the pKa for HCO3= 10.33. At pH = 9.0, what is the approximate relative quantities of H2CO3, HCO3-, and CO32-?
a. 1 H2CO3, 10 HCO3-, and 1 CO32-
b. Negligible H2CO3, 95 HCO3-, and 5 CO32-
c. 1 H2CO3, 90 HCO3-, and 10 CO32-
d. Negligible H2CO3, 75 HCO3-, and 25 CO32-

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Calculate the concentration of H+ ions in a 0.010 M aqueous solution of sulfuric acid.
Express your answer to three decimal places and include the appropriate units.

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Identify the weak diprotic acid.

A) CH3COOH

B) HCOOH

C) H3PO4

D) H2SO4

E) H2CO3

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