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Problem :

What is the pH of a solution of 40.0 g acetic acid, HAc, in 2.00 L of water? Acetic acid has a K a of 1.76 x 10-5.

Because acetic acid is a weak acid, we can make the assumption that x is much smaller than the initial concentration of HAc.

Problem :

Explain why HClO4 is more acidic than HClO3.

According to the Pauling-Ricci rule, pK a = 8 - 9f + 4n , f for HClO4 is 3 whereas HClO3 has f = 2. Conceptually we can understand why HClO4 has the lower pK a: the central atom Cl becomes more electronegative when it is adjoined to more electron-withdrawing oxygen atoms, and therefore the Cl atom withdraws electron density away from the oxygen holding the acidic proton. This weakens the bond between the oxygen and the proton.

Problem :

Why do you suppose H2S is a stronger acid than H2O?

The S-H bond in H2S is much weaker than the O-H bond in H2O because O is more electronegative than S. The weakly bonded proton is a mark of a stronger acid.

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