MCAT Chemical and Physical Foundations of Biological Systems Flashcards: 5a Acid Base Equilibria

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This deck focuses on 5a Acid Base Equilibria, giving you a quick way to review the definitions, rules, and examples that matter most for MCAT Chemical and Physical Foundations of Biological Systems.

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Work through these flashcards in short sessions. Try to answer each prompt before flipping the card, then revisit any cards you miss until the explanation feels automatic.

Flashcard 1: What is the Lewis definition of an acid?

Answer: An electron pair acceptor. Lewis theory defines acids as species that accept electron pairs to form coordinate bonds.

Flashcard 2: What is the relationship between conjugates: $K_a(\text{HA})K_b(\text{A}^-)$ at $25\,^{\circ}\text{C}$?

Answer: $K_aK_b=K_w$. For conjugate pairs, the product of Ka and Kb equals Kw at 25°C.

Flashcard 3: What is the formula for $K_b$ for $\text{B} + \text{H}_2\text{O} \rightleftharpoons \text{BH}^+ + \text{OH}^-$?

Answer: $K_b=\frac{[\text{BH}^+][\text{OH}^-]}{[\text{B}]}$. Kb expresses the equilibrium constant for weak base protonation, excluding water as a pure liquid.

Flashcard 4: What is the Henderson–Hasselbalch equation for a buffer of $\text{HA}/\text{A}^-$?

Answer: $pH=pK_a+\log\!\left(\frac{[\text{A}^-]}{[\text{HA}]}\right)$. The equation relates buffer pH to pKa and the ratio of conjugate base to acid.

Flashcard 5: Identify the major species at $pH=pK_a-2$ for a monoprotic acid system $\text{HA}/\text{A}^-$.

Answer: $\text{HA}$ predominates. At pH = pKa - 2, [A-]/[HA] = 0.01, so the protonated form is major.

Flashcard 6: What is the approximate buffer range (in pH units) around $pK_a$ for effective buffering?

Answer: $pK_a\pm^1$. Effective buffering occurs when [A-]/[HA] is between 0.1 and 10, spanning pKa ±1.

Flashcard 7: Which side is favored for $\text{HA}+\text{B}^-\rightleftharpoons \text{A}^-+\text{HB}$ when $pK_a(\text{HB})>pK_a(\text{HA})$?

Answer: Products (equilibrium favors the weaker acid). When pKa(HB) > pKa(HA), HB is weaker, so equilibrium shifts to the weaker acid side.

Flashcard 8: What is the conjugate base of the acid $\text{HA}$?

Answer: $\text{A}^-$. The conjugate base forms by removing a proton from the acid HA.

Flashcard 9: In a buffer, what is $pH$ when $[\text{A}^-]=[\text{HA}]$?

Answer: $pH=pK_a$. Equal concentrations give log(1) = 0, so pH equals pKa at half-equivalence.

Flashcard 10: What is the formula for percent ionization of a weak acid $\text{HA}$ in terms of $[\text{H}^+]$ and $[\text{HA}]_0$?

Answer: $\%\text{ionization}=\frac{[\text{H}^+]}{[\text{HA}]_0}\times100\%$. Percent ionization quantifies dissociation extent using equilibrium [H+] over initial [HA].

Flashcard 11: What is the relationship between $pOH$ and $[\text{OH}^-]$?

Answer: $pOH=-\log([\text{OH}^-])$. pOH measures basicity as the negative logarithm of hydroxide ion concentration.

Flashcard 12: At $25\,^{\circ}\text{C}$, what is the relationship between $pH$ and $pOH$?

Answer: $pH+pOH=14$. At 25°C, pH + pOH equals pKw = 14 from water's ion product.

Flashcard 13: Identify the major species at $pH=pK_a+2$ for a monoprotic acid system $\text{HA}/\text{A}^-$.

Answer: $\text{A}^-$ predominates. At pH = pKa + 2, [A-]/[HA] = 100, so the deprotonated form is major.

Flashcard 14: What is the Lewis definition of a base?

Answer: An electron pair donor. Lewis theory defines bases as species that donate electron pairs to form coordinate bonds.

Flashcard 15: At $25\,^{\circ}\text{C}$, what is $[\text{H}^+]$ in pure water?

Answer: $[\text{H}^+]=1.0\times10^{-7}\,\text{M}$. In pure water at 25°C, [H+] = [OH-] from autoionization, yielding neutral pH 7.

Flashcard 16: What is the relationship between $pK_a$ and $K_a$?

Answer: $pK_a=-\log(K_a)$. pKa is the negative logarithm of Ka, indicating acid strength inversely.

Flashcard 17: What is the relationship between $pK_b$ and $K_b$?

Answer: $pK_b=-\log(K_b)$. pKb is the negative logarithm of Kb, indicating base strength inversely.

Flashcard 18: What is the Brønsted–Lowry definition of a base?

Answer: A proton ($\text{H}^+$) acceptor. Brønsted–Lowry theory defines bases as species that accept protons from acids.

Flashcard 19: What is the value of $K_w$ at $25\,^{\circ}\text{C}$?

Answer: $K_w=1.0\times10^{-14}$. Kw is the ion product of water, constant at 25°C due to autoionization equilibrium.

Flashcard 20: What is the formula for $K_a$ for $\text{HA} + \text{H}_2\text{O} \rightleftharpoons \text{H}_3\text{O}^+ + \text{A}^-$?

Answer: $K_a=\frac{[\text{H}_3\text{O}^+][\text{A}^-]}{[\text{HA}]}$. Ka expresses the equilibrium constant for weak acid dissociation, excluding water as a pure liquid.

Flashcard 21: What is the net ionic equation for neutralization of a strong acid by a strong base in water?

Answer: $\text{H}^+ + \text{OH}^- \rightarrow \text{H}_2\text{O}$. Strong acid-base neutralization produces water by complete reaction of H+ and OH- ions.

Flashcard 22: What is the relationship between $pH$ and $[\text{H}^+]$?

Answer: $pH=-\log([\text{H}^+])$. pH measures acidity as the negative logarithm of hydrogen ion concentration.

Flashcard 23: What is the Brønsted–Lowry definition of an acid?

Answer: A proton ($\text{H}^+$) donor. Brønsted–Lowry theory defines acids as species that donate protons in reactions with bases.

Flashcard 24: What is the relationship between conjugates: $pK_a(\text{HA})+pK_b(\text{A}^-)$ at $25\,^{\circ}\text{C}$?

Answer: $pK_a+pK_b=14$. For conjugate pairs, pKa + pKb equals pKw = 14 at 25°C.

Flashcard 25: What is the conjugate acid of the base $\text{B}$?

Answer: $\text{BH}^+$. The conjugate acid forms by adding a proton to the base B.