(i) State the law of mass action.
(ii) What is a buffer solution? Give an example of an acidic buffer solution.
(iii) If the concentrations of ammonia and ammonium chloride in a buffer solution of ammonia–ammonium chloride are 0.2 M and 0.3 M respectively, determine the pH of the

Question

(i) State the law of mass action.
(ii) What is a buffer solution? Give an example of an acidic buffer solution.
(iii) If the concentrations of ammonia and ammonium chloride in a buffer solution of ammonia–ammonium chloride are 0.2 M and 0.3 M respectively, determine the pH of the buffer solution. (Given Kb(NH₃) = 1.76×10⁻⁵) [1 + 2 + 2]

**OR**

(i) Determine the pH of 0.1 M acetic acid solution. (pKa of acetic acid is 4.75) Is there any OH⁻ ion present in this solution of acetic acid? Answer with reason.
(ii) Why does the rate of dissociation of H₂S in aqueous solution decrease in the presence of HCl? [3 + 2]

Accepted Answer

>

> [!TLDR]
> Use the Henderson–Hasselbalch equation for a basic buffer, via pOH, to find the pH of the ammonia/ammonium chloride solution.
>
> **(i) Law of mass action:** the rate of a chemical reaction at a given temperature is directly proportional to the product of the **active masses** (molar concentrations) of the reacting substances, each raised to a power equal to its stoichiometric coefficient in the balanced equation.
>
> **(ii) Buffer solution:** a solution that resists a significant change in its pH when small amounts of acid or base are added to it, typically made of a weak acid and its conjugate base salt (acidic buffer) or a weak base and its conjugate acid salt (basic buffer). **Example of an acidic buffer:** a mixture of acetic acid ($CH_3COOH$) and sodium acetate ($CH_3COONa$).
>
> **(iii) pH of the $NH_3$–$NH_4Cl$ buffer:**
>
> Given: $[NH_3] = 0.2\ M$ (weak base), $[NH_4Cl] = 0.3\ M$ (salt providing conjugate acid $NH_4^+$), $K_b(NH_3) = 1.76	imes10^{-5}$.
> …

Q.(i) State the law of mass action.

Concept understanding — Buffer Solution pH

What is a Buffer Solution?

The Intuition: A Chemical Sponge

The Precise Statement: The Henderson–Hasselbalch Equation

Why This Works: A Quick Derivation

Example: Making an Acetate Buffer

Q.(i) State the law of mass action.

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