Topic 2: Water Chemistry 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?

• A.

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

   
• B.

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

   

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Optimal Buffering Range using pKa

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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.
Which one of the following pH values falls outside the optimal buffering rangeof Ethylenediaminetetraacetic acid?

• A. pH 2.1 
• B. pH 2.5 
• C. pH 7.4 
• D. pH 9.3 
• E. pH 10.4 

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Most Abundant Diprotic State at pH using pKa

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3706
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 8.5?

• A. C₅H₄N₄O₃ 
• B. C₅H₃N₄O₃^- 
• C. C₅H₂N₄O₃^2- 

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Most Abundant Triprotic State at pH using pKa

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Citric acid and its conjugate base, citrate, is an intermediate in the citric acid cycle.
Citric acid is triprotic with pK_a values of 3.13, 4.76, and 6.39.
Citric acid has four possible protonation states in the choices below.
Which one of the following protonation states is the most abundant at pH 2.5?

• A. HOC₃H₄(COOH)₃ 
• B. HOC₃H₄(COOH)₂(COO)^- 
• C. HOC₃H₄(COOH)(COO)₂^2- 
• D. HOC₃H₄(COO)₃^3- 

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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 10.5?

• A. H₄P₂O₇ 
• B. H₃P₂O₇^- 
• C. H₂P₂O₇^2- 
• D. HP₂O₇^3- 
• E. P₂O₇^4- 

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