Which acid is the strongest?

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Question: Which acid is the strongest?

Option 1: HF (Ka ~ 10^-3, pKa ~ 3). This is a very weak acid in water, since a Ka of 10^-3 corresponds to a pKa around 3. In aqueous solution, such a small dissociation constant means little ionization, so HF is not the strongest.

Option 2: HCl (Ka ~ 10^7, pKa ~ -7). A Ka on the order of 10^7 indicates a very strong acid in water, with substantial proton donation. However, it must be compared to the other candidates; its strength is high but not the maximum across the listed acids.

Option 3: HBr (Ka ~ 10^9, pKa ~ -9). This is stronger than HCl (larger Ka, more negative pKa), since greater Ka means greater dissociation in water and thus a stronger acid.

Option 4: HI (Ka ~ 10^11, pKa ~ -11). This has the largest Ka among the options and the most negative pKa, indicating the greatest tendency to donate a proton in aqueous solution.

Reasoning about acid strength: In aqueous solutions, the strength of an acid is quantified by Ka (or pKa). A larger Ka (or more negative pKa) means greater acid strength because the acid more completely dissociates to H+ and its conjugate base. Down the halogen group, bond strength weakens and the conjugate base becomes more stable as the hydrogen-halide bond weakens, contributing to higher acidity. Therefore, the trend among these hydrohalic acids is HF < HCl < HBr < HI in strength, with HI being the strongest of the four.

Why the other choices are not the strongest: HF has the smallest Ka, so it is weak and far from the strongest. HCl, while extremely acidic, has a smaller Ka than HBr and HI, so it is not the strongest among the listed acids. HBr is stronger than HCl but still weaker than HI, given its smaller Ka. HI shows the largest Ka value, consistent with the strongest acid in this set.

Summary of key concepts: acid strength in water correlates with Ka (or pKa); larger Ka and more negative pKa denote stronger acids; among hydrohalic acids, strength increases from HF to HI due to bond dissociation energies and conjugate base stability.

Chem 1C General Chemis... Self Assessment Quiz 12

Which acid is the strongest?

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