30. When a small amount of 12 M HNO3(aq) is added to a buffer solution made by mixing CH3NH2(aq) and — Acids and Bases Chemistry Question

STEPS:

1. Identify the components of the buffer system: The buffer solution is prepared by mixing the weak base methylamine ($\text{CH}_3\text{NH}_2$) and its conjugate acid, the methylammonium ion ($\text{CH}_3\text{NH}_3^+$), which is provided by the soluble salt methylammonium chloride ($\text{CH}_3\text{NH}_3\text{Cl}$).
2. Analyze the species introduced by the added strong acid: Nitric acid ($\text{HNO}_3$) is a strong acid that dissociates completely in aqueous solutions to release hydrogen ions ($\text{H}^+$) into the system.
3. Determine the buffering reaction: When a strong acid is added to a buffer, the weak base component of the buffer system reacts to consume the added $\text{H}^+$ ions.
4. Formulate the reaction equation: The weak base methylamine ($\text{CH}_3\text{NH}_2$) acts as a proton acceptor, neutralizing the added $\text{H}^+$ and converting it to the weak conjugate acid methylammonium ($\text{CH}_3\text{NH}_3^+$):
$$\text{CH}_3\text{NH}_2(aq) + \text{H}^+(aq) \rightarrow \text{CH}_3\text{NH}_3^+(aq)$$
5. Relate the reaction to the pH stability: Because the added strong $\text{H}^+$ ions are successfully converted into a weak conjugate acid, the concentration of free $\text{H}^+$ in the solution increases only slightly, which keeps the pH from changing significantly (only dropping from 10.64 to 10.62). This reaction perfectly matches Option A.

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WHY_OTHERS_WRONG:

• Option B is incorrect: The methylammonium ion ($\text{CH}_3\text{NH}_3^+$) is already the conjugate acid of the system. Due to positive-charge repulsion, it will not accept another proton under these conditions to form a highly unstable divalent ion ($\text{CH}_3\text{NH}_4^{2+}$).
• Option C is incorrect: Because nitric acid ($\text{HNO}_3$) is a strong acid, its conjugate base ($\text{NO}_3^-$) is extremely weak and has negligible basicity. Nitrate will not react with $\text{H}^+$ to reform molecular $\text{HNO}_3$ in an aqueous solution.
• Option D is incorrect: While $\text{H}^+$ can react with free hydroxide ions ($\text{OH}^-$) to form water, the concentration of free hydroxide ions in this buffer is very low. The principal component responsible for the buffering capacity against added acid is the weak base $\text{CH}_3\text{NH}_2$ itself, which is present in a much higher concentration.