A student adds 0.50 mL of 1.0 M HCl to 100 mL of a buffer that is 0.20 M in both…

CHEMISTRY · Single correct · A Level · Chemistry · Acids and bases · Acids and bases

Problem

A student adds 0.50 mL of 1.0 M HCl to 100 mL of a buffer that is 0.20 M in both acetic acid (CH3COOH\text{CH}_3\text{COOH}) and sodium acetate (CH3COONa\text{CH}_3\text{COONa}). Which statement best explains why the pH decreases only slightly after the addition? (a) Because only 0.50 mL of acid is added to 100 mL of buffer, the effect is mainly a dilution that hardly alters [H+][\text{H}^+]. (b) The added H+H^+ ions are neutralized by hydroxide ions from water auto-ionization, so very little net acid remains. (c) The added H+H^+ is consumed when it protonates CH3COO−\text{CH}_3\text{COO}^-, so the ratio [base]/[acid][\text{base}]/[\text{acid}] changes very little. (d) When [CH3COOH]=[CH3COO−][\text{CH}_3\text{COOH}]=[\text{CH}_3\text{COO}^-], the Henderson–Hasselbalch equation fixes the pH at pKapK_a no matter how much strong acid is added.

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