2: Water and pH

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

LibreTexts reference: Chemistry of Water

Correct Henderson-Hasselbalch Equation

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

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

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

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pH Using the Henderson-Hasselbalch Equation

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Use the Henderson-Hasselbalch equation to calculate the pH.
A solution contains 6.5 g of HC₂H₃O₂ and 13.5 g of NaC₂H₃O₂ in 118.2 mL of solution.
(pK_a of acetic acid = 4.76)
Molar masses: HC₂H₃O₂ = 60.05 g/mol; NaC₂H₃O₂ = 82.03 g/mol.
Henderson-Hasselbalch equation:
$\mathrm{pH}\hspace{0.17em}=\hspace{0.17em}{\mathrm{pK}}_a +\hspace{0.17em}{log}_{10}\left(\frac{\left[\mathrm{Base}\right]}{\left[\mathrm{Acid}\right]}\right)$

• 5.14
• 4.94
• 4.59
• 4.34

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pH Using the Henderson-Hasselbalch Equation

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Calculate the pH of the buffer solution.
The solution contains 104.0 mM of butyric acid and 512.0 mM of sodium butyrate.
(pK_a of butyric acid = 4.82)
Henderson-Hasselbalch equation:
$\mathrm{pH}\hspace{0.17em}=\hspace{0.17em}{\mathrm{pK}}_a +\hspace{0.17em}{log}_{10}\left(\frac{\left[\mathrm{Base}\right]}{\left[\mathrm{Acid}\right]}\right)$
Note: answers need to be within 1% of the correct number to be correct.

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pKa and pKb Using the Henderson-Hasselbalch Equation

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Calculate pK_a for the weak acid.
The solution contains 516.0 mM of acetic acid and 313.0 mM of sodium acetate.
The measured pH of the solution is 4.54.
Henderson-Hasselbalch equation:
$\mathrm{pH}\hspace{0.17em}=\hspace{0.17em}{\mathrm{pK}}_a +\hspace{0.17em}{log}_{10}\left(\frac{\left[\mathrm{Base}\right]}{\left[\mathrm{Acid}\right]}\right)$

• 4.56
• 4.96
• 4.41
• 4.76

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pKa and pKb Using the Henderson-Hasselbalch Equation

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Calculate pK_b for the weak base.
The solution contains 49.6 mM of ammonia and 32.4 mM of ammonium chloride.
The measured pH of the solution is 9.42.
Hint: pK_a + pK_b = 14.00 for a conjugate acid-base pair.
Henderson-Hasselbalch form:
$\mathrm{pH}\hspace{0.17em}=\hspace{0.17em}{\mathrm{pK}}_a +\hspace{0.17em}{log}_{10}\left(\frac{\left[\mathrm{Base}\right]}{\left[\mathrm{Acid}\right]}\right)$
Note: answers need to be within 2% of the correct number to be correct.

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Acid-Base Ratios Using the Henderson-Hasselbalch Equation

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Calculate the ratio of conjugate base to weak acid.
For acetic acid / acetate, pK_a = 4.76 and the desired pH is 4.12.
What is the ratio [A^-] / [HA] ?

•  0.458
•  0.229
•  4.365
•  0.023

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Conjugate Base-Acid Ratios Using the Henderson-Hasselbalch Equation

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Calculate the ratio of conjugate base to weak acid.
For acetic acid / acetate, pK_a = 4.76 and the desired pH is 5.17.
What is the ratio [A^-] / [HA] ?
Note: answers need to be within 6% of the correct number to be correct.

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Protonation States of Functional Groups at Different pH Levels

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When the pH is more than two (2) pH units ABOVE the pKa of a amine group (e.g., -NH₂), what form is the chemical group in?
Select BOTH the number of hydrogens and its charge; check two boxes.

• no hydrogens (0 H)
• one hydrogen (1 H)
• two hydrogens (2 H)
• three hydrogens (3 H)
• positive charge (+)
• neutral charge (0)
• negative charge (-)

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Functional Groups with Single Bonds

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Which one of the following sets of three (3) functional groups all contain a double bond?

• carboxyl, carbonyl, hydroxyl
• hydroxyl, carboxyl, phosphate
• sulfhydryl, methyl, phosphate
• sulfhydryl, carboxyl, phosphate
• carboxyl, phosphate, carbonyl

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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?

• pH 1.8
• pH 2.8
• pH 5.9
• pH 9.6
• pH 11.7

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The Most Abundant Diprotic State at a Given pH Using pKa

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Carbonic acid and its conjugate base, carbonate, is also known as respiratory acid as it is the only acid excreted as a gas by the lungs.
Carbonic acid is diprotic with pK_a values of 6.35 and 10.33.
Carbonic acid has three possible protonation states in the choices below.
Which one of the following protonation states is the most abundant at pH 9.0?

• H₂CO₃
• HCO₃^-
• CO₃^2-

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The Most Abundant Triprotic State at a Given 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 5.5?

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

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The Most Abundant Tetraprotic State at a Given 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 5.5?

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

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pH Differences Between Solutions

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The pH of the lemon juice is 1.9, while the battery acid is pH 0.9. This is a difference of 1 pH units.
The battery acid has:

• 1 times higher [H+] than the lemon juice.
• 10 (10¹) times higher [H+] than the lemon juice.
• 10 (10¹) times lower [H+] than the lemon juice.
• 1 times lower [H+] than the lemon juice.

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