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?

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

   

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

   

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Optimal Buffering Range using pKa

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

• pH 4.2 
• pH 5.5 
• pH 5.7 
• pH 7.0 
• pH 8.2 

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Most Abundant Diprotic State at pH using pKa

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Sulfurous acid and its conjugate base, sulfite, occurs naturally in wine, but larger amounts are added to wine to stop fermentation and prevent spoilage.
Sulfurous acid is diprotic with pK_a values of 1.81 and 6.97.
Sulfurous 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?

• SO₃^2-
• HSO₃^-
• H₂SO₃

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Most Abundant Triprotic State at pH 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.
Phosphoric acid has four possible protonation states in the choices below.
Which one of the following protonation states is the most abundant at pH 3.0?

• H₃PO₄
• H₂PO₄^-
• HPO₄^2-
• PO₄^3-

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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?

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

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