Acids, Bases and Salts

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Acids, Bases and Salts — Key Facts for JAMB

Arrhenius Definitions:

• Acid: produces $H^+$ ions in aqueous solution (e.g., $HCl → H^+ + Cl^-$)
• Base: produces $OH^-$ ions in aqueous solution (e.g., $NaOH → Na^+ + OH^-$)

Bronsted-Lowry Definitions:

• Acid: $H^+$ donor
• Base: $H^+$ acceptor
• Conjugate acid-base pair: differ by one proton ($HCl/Cl^-$, $NH_4^+/NH_3$)
• The stronger the acid, the weaker its conjugate base

pH Scale: $pH = -\log[H^+]$. At $25°C$:

• Neutral: $[H^+] = 10^{-7}$ M, $pH = 7$
• Acidic: $[H^+] > 10^{-7}$ M, $pH < 7$
• Basic: $[H^+] < 10^{-7}$ M, $pH > 7$
• $pOH = -\log[OH^-]$, $pH + pOH = 14$

Salt Hydrolysis:

• Salt of strong acid + strong base: neutral ($pH = 7$), e.g., NaCl
• Salt of weak acid + strong base: basic ($pH > 7$), e.g., $CH_3COONa$: $CH_3COO^- + H_2O \rightleftharpoons CH_3COOH + OH^-$
• Salt of strong acid + weak base: acidic ($pH < 7$), e.g., $NH_4Cl$: $NH_4^+ + H_2O \rightleftharpoons NH_3 + H_3O^+$

⚡ Exam tip: The $pH$ of a salt solution tells you which is stronger — the parent acid or the parent base. A salt of a weak acid and weak base has $pH$ dependent on $K_a$ and $K_b$.

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Acids, Bases and Salts — JAMB UTME Study Guide

Strong vs Weak Acids:

• Strong acids: $HCl$, $HBr$, $HI$, $HNO_3$, $H_2SO_4$ (first proton), $HClO_4$. Completely dissociated in water.
• Weak acids: $CH_3COOH$, $H_2CO_3$, $H_3PO_4$, $HF$, $H_2SO_3$. Partially dissociated: $K_a = [H^+][A^-]/[HA]$.
• $K_a$ for acetic acid = $1.8 \times 10^{-5}$. For a 0.1 M solution: $[H^+] = \sqrt{K_a C} = \sqrt{1.8 \times 10^{-6}} = 1.34 \times 10^{-3}$ M. $pH = 2.87$.

Strong vs Weak Bases:

• Strong bases: $NaOH$, $KOH$, $Ba(OH)_2$ (2 OH⁻ per formula unit). Completely dissociated.
• Weak bases: $NH_3$ ($K_b = 1.8 \times 10^{-5}$), amines. $K_b = [BH^+][OH^-]/[B]$.

Buffer Solutions: A buffer resists pH change on addition of small amounts of acid or base. Made from:

• Weak acid + its conjugate base salt: $CH_3COOH/CH_3COONa$
• Weak base + its conjugate acid salt: $NH_3/NH_4Cl$

Henderson-Hasselbalch equation: $pH = pK_a + \log([A^-]/[HA])$.

Neutralisation Reactions:

• Acid + metal hydroxide → salt + water: $HCl + NaOH → NaCl + H_2O$
• Acid + metal carbonate → salt + water + CO₂: $2HCl + Na_2CO_3 → 2NaCl + H_2O + CO_2$
• Acid + metal bicarbonate → salt + water + CO₂: $HCl + NaHCO_3 → NaCl + H_2O + CO_2$

Solubility Rules:

• Nitrates ($NO_3^-$): ALL soluble
• Group 1 and ammonium ($NH_4^+$): ALL soluble
• Chlorides: soluble EXCEPT $AgCl$, $PbCl_2$ (sparingly soluble, soluble in hot water)
• Sulfates: soluble EXCEPT $BaSO_4$, $PbSO_4$, $CaSO_4$ (slightly)
• Carbonates, phosphates, sulfides, hydroxides: insoluble EXCEPT Group 1 and ammonium

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Acids, Bases and Salts — Comprehensive Chemistry Notes

pH Calculations — Systematic Approach:

For strong acid $HCl$ of concentration $C$: $[H^+] = C$ (complete dissociation). For $0.01$ M $HCl$: $[H^+] = 10^{-2}$ M. $pH = 2$.

For weak acid $HA$: Write dissociation equation, ICE table, solve quadratic: $HA \rightleftharpoons H^+ + A^-$; $K_a = x^2/(C - x)$. If $K_a \ll C$, $x \approx \sqrt{K_a C}$.

For polyprotic acids like $H_2SO_4$: First proton is strong ($K_{a1}$ is large). Second proton is weak: $HSo_4^- \rightleftharpoons H^+ + SO_4^{2-}$, $K_{a2} = 1.2 \times 10^{-2}$. For $0.1$ M $H_2SO_4$: first $H^+$ gives $[H^+] = 0.1$ M. Second dissociation adds $x$ where $x^2/(0.1 - x) = 0.012$. Solving: $x = 0.0155$. Total $[H^+] = 0.1155$ M. $pH = 0.937$.

Lewis Theory:

• Acid: electron pair acceptor
• Base: electron pair donor
• Examples: $AlCl_3$ (Lewis acid) accepts a pair from $NH_3$ (Lewis base) → $AlCl_3·NH_3$

Ionic Product of Water: $K_w = [H^+][OH^-] = 10^{-14}$ at $25°C$. This varies with temperature: at $0°C$, $K_w ≈ 10^{-15}$; at $100°C$, $K_w ≈ 10^{-12}$. At $100°C$, neutral $pH = 6$ (not 7).

Salt of Weak Acid + Weak Base — pH: For $CH_3COONH_4$: both ions hydrolyse. Net effect: $pH = \frac{1}{2}(pK_a + pK_b)$. If $K_a = K_b$, the solution is neutral. Otherwise, compare $K_a$ and $K_b$.

Buffer Capacity: Buffer capacity = number of moles of strong acid/base needed to change pH by 1 unit per litre of buffer. Maximum when $[HA] = [A^-]$, i.e., $pH = pK_a$.

Ostwald Dilution Law: For weak electrolyte: $\alpha = \sqrt{K/C}$ where $\alpha$ = degree of dissociation. As concentration $C$ decreases, $\alpha$ increases. This is why diluting acetic acid increases its degree of dissociation (though $[H^+]$ might decrease or increase depending on concentration).

Indicators:

• Methyl orange: $pH$ range 3.1–4.4 (red in acid, yellow in base). Suitable for strong acid + weak base titrations.
• Phenolphthalein: $pH$ range 8.2–10.0 (colourless in acid, pink in base). Suitable for strong acid + strong base titrations.
• Litmus: $pH$ range 5.5–8.2 (red in acid, blue in base). Used for detecting acids/bases generally.

Equivalent Weight:

• Acid: $E = M/valence$ (number of $H^+$ per molecule)
• Base: $E = M/valence$ (number of $OH^-$ per formula unit)
• Salt: $E = M/total\ positive\ charge$
• For $H_2SO_4$: $E = 98/2 = 49$. For $Al_2(SO_4)_3$: $E = 342/6 = 57$.

JAMB Pattern Analysis: Common questions: (1) $pH$ of strong acid/weak acid solutions, (2) Salt hydrolysis and $pH$, (3) Buffer preparation using Henderson-Hasselbalch, (4) Titration curves (what indicator for what type of titration), (5) $K_w$ at different temperatures. JAMB 2023: “Calculate the $pH$ of $0.01$ M $NaOH$ solution.” Answer: $[OH^-] = 10^{-2}$, $pOH = 2$, $pH = 14 - 2 = 12$.

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📊 JAMB Exam Essentials

Detail	Value
Questions	180 MCQs (UTME)
Subjects	4 subjects (language + 3 for course)
Time	2 hours
Marking	+1 per correct answer
Score	400 max (used for university admission)
Registration	January – February each year

🎯 High-Yield Topics for JAMB

• Use of English (Grammar + Comprehension) — 60 marks
• Biology for Science students — 40 marks
• Chemistry (Organic + Physical) — 40 marks
• Physics (Mechanics + Optics) — 35 marks
• Mathematics (Algebra + Geometry) — 40 marks

📝 Previous Year Question Patterns

• Q: “The process of photosynthesis requires…” [2024 Biology]
• Q: “The electronic configuration of Fe is…” [2024 Chemistry]
• Q: “Find the value of x if 2x + 5 = 15…” [2024 Mathematics]

💡 Pro Tips

• Use of English carries the most weight — master grammar rules and comprehension strategies
• JAMB syllabus is your Bible — questions come directly from it. Download and use it.
• Past questions are highly predictive — repeat patterns appear every year
• For Science students, Biology and Chemistry are high-scoring if you study NCERT-level content

🔗 Official Resources

• JAMB Official
• JAMB Syllabus

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