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7
Neutral pure water has equal H+ and OH− activities; pH is approximately 7 at 25°C.
Class 12 · Bangla Version · Chemistry
Reactions, acids, bases and salts
Explain acid-base reactions, equilibrium and the limits of a pH comparison.
Shared cross-level core and stretch unit. Start with the core questions, then use the stretch example for deeper work. The same unit is available in related class and board pathways; it is not a full grade-specific or examination-specification course. A teacher should select the appropriate depth and further topics.
Learn the idea
An acid donates a proton in the Brønsted model; a base accepts one. Strong and concentrated describe different things: strength concerns extent of ionisation, concentration concerns amount per volume. At equilibrium, forward and reverse rates are equal, so concentrations stay constant without reactions stopping.
Worked example
HCl + NaOH → NaCl + H2O represents neutralisation. For a strong acid with [H+] = 0.001 mol/dm³ in the simplified dilute model, pH = −log10[H+] = 3.
- Identify the acid-base roles.
- Separate strength from concentration.
- State the conditions behind a pH or equilibrium calculation.
Stretch example
For HA ⇌ H+ + A−, Ka = [H+][A−]/[HA] under the concentration approximation. A buffer contains a weak acid and its conjugate base; it resists, rather than prevents, pH changes from small additions.
Your written practice
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sodium chloride
Na+ and Cl− remain in solution.
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2
−log10(0.01) = 2.
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Ionisation and concentration both matter, so the labels alone do not determine [H+].
Do not equate strong with concentrated.
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Forward and reverse reactions continue at equal rates.
State continuing reaction and stable macroscopic amounts.
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1
The scale uses a base-ten logarithm.
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A sufficiently large addition consumes much of one component, so the acid/base pair can no longer absorb the change effectively.
Describe consumption of buffer components.
Paper comparison
Compare a strong dilute and weak concentrated acid using supplied fictional concentrations.
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- State which quantities are given.
- Do not infer pH from labels alone.
- Use calculations only; do not mix chemicals.
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