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Question
The pKa value of HCl is −7.0 and that of HI is −10.0. Which is the stronger acid?
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Solution
pKa is the negative logarithm of the acid dissociation constant (Ka):
pKa = −log10(Ka)
Lower pKa ⇒ higher Ka ⇒ stronger acid
pKa of HCl = −7.0
pKa of HI = −10.0
Since −10 < −7, HI has a lower pKa, so it has a higher Ka.
The smaller the pKa value, the greater its acidity. Hence, HI is a stronger acid than HCl.
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