2: Water and pH

Properties of water, pH, pK_a, and buffering systems.

LibreTexts reference: Chemistry of Water

Correct Form of the Henderson-Hasselbalch Equation

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Which one of the following equations is the correct form of the Henderson-Hasselbalch equation?

• $\mathrm{pH}\hspace{0.17em}=\hspace{0.17em}{\mathrm{pK}}_a +\hspace{0.17em}{log}_{10}\left(\frac{\left[{\mathrm{A}}^{–}\right]}{\left[\mathrm{HA}\right]}\right)$

   

• $\mathrm{pH}\hspace{0.17em}=\hspace{0.17em}{\mathrm{pK}}_a +\hspace{0.17em}{log}_{10}\left(\frac{\left[\mathrm{HA}\right]}{\left[{\mathrm{A}}^{–}\right]}\right)$

   

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Optimal Buffering Range using pKa

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Phosphoric acid and its conjugate base, phosphate, is abbreviated P_i and is formed by cells from the hydrolysis of ATP into ADP.
Phosphoric acid is triprotic with pK_a values of 2.16, 7.21, and 12.32.
Which one of the following pH values falls outside the optimal buffering rangeof Phosphoric acid?

• pH 3.1 
• pH 6.2 
• pH 8.0 
• pH 10.4 
• pH 13.2 

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Most Abundant Diprotic State at pH using pKa

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Uric acid and its conjugate base, urate, is an end product of nucleotide purine metabolism..
Uric acid is diprotic with pK_a values of 5.40 and 10.30.
Uric acid has three possible protonation states in the choices below.
Which one of the following protonation states is the most abundant at pH 11.5?

• C₅H₄N₄O₃
• C₅H₃N₄O₃^-
• C₅H₂N₄O₃^2-

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Most Abundant Triprotic State at pH using pKa

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Arsenic acid and its conjugate base, arsenate, is extremely toxic, corrosive, and carcinogenic; it serves as a precursor to a variety of pesticides.
Arsenic acid is triprotic with pK_a values of 2.19, 6.94, and 11.50.
Arsenic acid has four possible protonation states in the choices below.
Which one of the following protonation states is the most abundant at pH 5.0?

• AsO₄^3-
• HAsO₄^2-
• H₂AsO₄^-
• H₃AsO₄

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Most Abundant Tetraprotic State at pH using pKa

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Ethylenediaminetetraacetic acid and its conjugate base, EDTA, is commonly used in chelation therapy and as a chelating agent in biochemical experiments..
Ethylenediaminetetraacetic acid is tetraprotic with pK_a values of 2.00, 2.67, 6.16, and 10.26.
Ethylenediaminetetraacetic acid has five possible protonation states in the choices below.
Which one of the following protonation states is the most abundant at pH 4.5?

• C₁₀H₁₂N₂O₈^4-
• C₁₀H₁₃N₂O₈^3-
• C₁₀H₁₄N₂O₈^2-
• C₁₀H₁₅N₂O₈^-
• C₁₀H₁₆N₂O₈

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