Which of the following is the weakest acid?

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Question restatement: Which of the following is the weakest acid?

Option A: H3BO3 (Ka for H3BO3 is 5.4 × 10^-10)
- Analysis: This value indicates a very weak acid, since the acid dissociation constant is on the order of 10^-10. Among the given options, this Ka is larger than 4.9 × 10^-10 but still extremely small, which means it is not as strong an acid as HF or HCNO. However, its Ka is not the smallest overall here.

Option B: HCN (Ka for HCN is 4.9 × 10^-10)
- Analysis: This Ka is slightly smaller than 5.4 × 10^-10, placing HCN as a very weak acid—indeed weaker than H3BO3 in this set because a smaller Ka corresponds to weaker acidity. Since the task is to identify the weakest acid, this is a strong candidate for the correct choice.

Option C: HF (Ka for HF is 7.2 × 10^-4)
- Analysis: A Ka on the order of 10^-4 to 10^-3 indicates a relatively strong acid in aqueous solution. HF is significantly stronger than the previous two options, so this option is not the weakest.

Option D: HCNO (Ka for HCNO is 2 × 10^-4)
- Analysis: With a Ka around 10^-4, HCNO is also a relatively stronger acid compared to HCN and H3BO3, though weaker than HF. It is not the weakest among the choices.

Putting the comparisons together: The strength of an acid in solution is primarily determined by its Ka value; the smaller the Ka, the weaker the acid. Between the options with Ka values in the 10^-10 range, HCN has the smaller value (4.9 × 10^-10) than H3BO3 (5.4 × 10^-10), HCNO (2 × 10^-4), or HF (7.2 × 10^-4). Therefore, HCN is the weakest acid among the listed substances.

Additional nuance: While both H3BO3 and HCN are very weak acids, the given numeric Ka shows HCN being the weaker of the two in this problem setup. HF and HCNO are clearly much stronger acids due to their substantially larger Ka values, so they are not candidates for the weakest.

SP25: CHEM-30A Sec 01 - Intro Chem Final Exam

Which of the following is the weakest acid?

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