2: Water and pH

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

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?

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

   

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

   

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Optimal Buffering Range using pKa

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Pyrophosphoric acid and its conjugate base, pyrophosphate, is abbreviated PP_i and is formed by cells from the hydrolysis of ATP into AMP.
Pyrophosphoric acid is tetraprotic with pK_a values of 0.91, 2.10, 6.70, and 9.32.
Which one of the following pH values falls outside the optimal buffering rangeof Pyrophosphoric acid?

• pH 0.6 
• pH 1.3 
• pH 8.6 
• pH 10.2 
• pH 10.6 

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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 4.0?

• C₅H₂N₄O₃^2-
• C₅H₃N₄O₃^-
• C₅H₄N₄O₃

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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 12.5?

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

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Most Abundant Tetraprotic State at pH using pKa

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Pyrophosphoric acid and its conjugate base, pyrophosphate, is abbreviated PP_i and is formed by cells from the hydrolysis of ATP into AMP.
Pyrophosphoric acid is tetraprotic with pK_a values of 0.91, 2.10, 6.70, and 9.32.
Pyrophosphoric acid has five possible protonation states in the choices below.
Which one of the following protonation states is the most abundant at pH 7.5?

• H₄P₂O₇
• H₃P₂O₇^-
• H₂P₂O₇^2-
• HP₂O₇^3-
• P₂O₇^4-

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