Acids and Bases — Hard Practice Quiz
A Chemistry cheat sheet for Acids and Bases — every key formula with its symbols defined — plus a hard-level practice quiz to test recall.
Formulas & key concepts
Conjugate Acid-Base Pairs: Proton transfer between acid HX and water.
Proton Transfer in Water: If \(H_2O\) is a stronger base than \(X^-\), equilibrium favors products.
Acid Dissociation Constant: Equilibrium constant for weak acid ionization.
Where: \(K_a\) = acid dissociation constant
pH Definition: Negative logarithm of hydrogen ion concentration.
pOH Definition: Negative logarithm of hydroxide ion concentration.
pH-pOH Relationship: Sum of pH and pOH equals 14 at 25°C.
H+ from pH: Calculate hydrogen ion concentration from pH.
OH- from pOH: Calculate hydroxide ion concentration from pOH.
Percent Ionization: Fraction of acid molecules that donate protons.
Weak Acid Ionization: General equation for weak acid dissociation.
Weak Acid Ionization Simplified: Simplified form without explicit water.
Weak Acid Ka Expression: Equilibrium constant expression for weak acid.
Weak Base Ionization: Base accepts proton from water.
Base Dissociation Constant: Equilibrium constant for weak base ionization.
Where: \(K_b\) = base dissociation constant
Ka-Kb Relationship: Product of acid and conjugate base constants equals \(K_w\) at 25°C.
pKa-pKb Relationship: Sum of pKa and pKb of conjugate pair equals 14 at 25°C.
Conjugate Base Reaction: Weak conjugate base reacts with water to produce weak acid and hydroxide.
| Acid | Base |
|---|---|
| HCl (strong) | Cl⁻ (negligible) |
| H₂SO₄ (strong) | HSO₄⁻ (weak) |
| HNO₃ (strong) | NO₃⁻ (negligible) |
| H₃O⁺ | H₂O |
| HF (weak) | F⁻ (weak) |
| CH₃COOH (weak) | CH₃COO⁻ (weak) |
| NH₄⁺ (weak) | NH₃ (weak) |
| H₂O | OH⁻ (strong) |
| Acid | Ka₁ | Ka₂ | Ka₃ |
|---|---|---|---|
| H₃PO₄ (Phosphoric) | 7.5×10⁻³ | 6.2×10⁻⁸ | 4.2×10⁻¹³ |
| H₂SO₃ (Sulfurous) | 1.7×10⁻² | 6.4×10⁻⁸ | — |
| H₂CO₃ (Carbonic) | 4.3×10⁻⁷ | 5.6×10⁻¹¹ | — |
| H₂C₂O₄ (Oxalic) | 5.9×10⁻² | 6.4×10⁻⁵ | — |
| Acid | Ka | Base | Kb |
|---|---|---|---|
| HF | 6.8×10⁻⁴ | F⁻ | 1.5×10⁻¹¹ |
| HC₂H₃O₂ | 1.8×10⁻⁵ | C₂H₃O₂⁻ | 5.6×10⁻¹⁰ |
| NH₄⁺ | 5.6×10⁻¹⁰ | NH₃ | 1.8×10⁻⁵ |
| HCO₃⁻ | 5.6×10⁻¹¹ | CO₃²⁻ | 1.8×10⁻⁴ |
Practice quiz
A $0.10 \text{ M}$ solution of a weak acid $HA$ has a $K_a$ of $1.8 \times 10^{-5}$. Calculate the pH of the solution and the percent ionization of the acid.
- pH = $2.87$, Percent ionization = $1.34\%$
- pH = $1.00$, Percent ionization = $18\%$
- pH = $4.74$, Percent ionization = $0.018\%$
- pH = $2.87$, Percent ionization = $0.134\%$
Answer: pH = $2.87$, Percent ionization = $1.34\%$
A $0.25 \text{ M}$ solution of a weak base $B$ has a pH of $11.20$. Determine the $K_b$ for this base.
- $1.0 \times 10^{-5}$
- $1.8 \times 10^{-5}$
- $2.5 \times 10^{-3}$
- $6.3 \times 10^{-12}$
Answer: $1.0 \times 10^{-5}$
Acetic acid ($CH_3COOH$) has a $K_a$ of $1.8 \times 10^{-5}$. What is the pH of a $0.15 \text{ M}$ solution of sodium acetate ($CH_3COONa$), which is the conjugate base?
- $4.74$
- $7.00$
- $8.96$
- $11.26$
Answer: $8.96$
Calculate the pH of a $0.10 \text{ M}$ solution of phosphoric acid ($H_3PO_4$). Use the provided table for $K_a$ values and assume only the first dissociation is significant.
- $1.20$
- $1.62$
- $2.12$
- $3.00$
Answer: $1.62$
A $0.10 \text{ M}$ solution of a weak acid $HA$ is diluted to $0.010 \text{ M}$. How does this dilution affect the percent ionization of the acid and the pH of the solution?
- Percent ionization decreases, pH decreases.
- Percent ionization increases, pH increases.
- Percent ionization increases, pH decreases.
- Percent ionization decreases, pH increases.
Answer: Percent ionization increases, pH increases.
A weak acid $HA$ has a $K_a$ of $4.0 \times 10^{-7}$. What is the ratio of the conjugate base concentration to the weak acid concentration, $\frac{[A^-]}{[HA]}$, when the solution has a pH of $6.00$?
- $0.040$
- $0.40$
- $2.5$
- $4.0$
Answer: $0.40$
$50.0 \text{ mL}$ of $0.20 \text{ M}$ $HCl$ is mixed with $150.0 \text{ mL}$ of $0.050 \text{ M}$ $NaOH$. What is the pH of the resulting solution?
- $1.90$
- $7.00$
- $12.10$
- $1.00$
Answer: $1.90$
Two weak acids, $HA$ and $HB$, are prepared at the same initial concentration of $0.10 \text{ M}$. Solution $HA$ has a pH of $3.00$, while solution $HB$ has a pH of $4.00$. Which statement is true regarding their relative strengths and their conjugate bases?
- $HA$ is a stronger acid than $HB$, and $A^-$ is a stronger base than $B^-$.
- $HA$ is a stronger acid than $HB$, and $A^-$ is a weaker base than $B^-$.
- $HB$ is a stronger acid than $HA$, and $B^-$ is a stronger base than $A^-$.
- $HB$ is a stronger acid than $HA$, and $B^-$ is a weaker base than $A^-$.
Answer: $HA$ is a stronger acid than $HB$, and $A^-$ is a weaker base than $B^-$.
A weak acid $HX$ has a $K_a$ of $2.0 \times 10^{-3}$. If the initial concentration of $HX$ is $0.050 \text{ M}$, calculate its percent ionization. Is the approximation that $x$ is negligible compared to the initial concentration valid in this case?
- $1.0\%$; Approximation is valid.
- $2.0\%$; Approximation is valid.
- $18.1\%$; Approximation is not valid.
- $20.0\%$; Approximation is not valid.
Answer: $18.1\%$; Approximation is not valid.
A solution contains $0.10 \text{ M}$ acetic acid ($CH_3COOH$) and $0.20 \text{ M}$ sodium acetate ($CH_3COONa$). Given that the $K_a$ for acetic acid is $1.8 \times 10^{-5}$, calculate the pH of this solution.
- $4.44$
- $4.74$
- $5.05$
- $5.35$
Answer: $5.05$
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