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1. Compare HCl with H2S: HCl is a stronger acid than H2S. Acids become stronger as the X-H bond becomes more polar. The larger the difference in the electronegativity between X and H atoms, the easier it is to form H + and X - ions.
2. Compare H2S with distilled water: H2S is a stronger acid than water. Although the H-S bond is not as polar as the H-O bond, H2S is a stronger acid because of the size of the sulfur atom compared to that of oxygen. The bond dissociation energy associated with larger atoms is greater than that of a smaller atoms because, given equal charges, the charge density is greater on the smaller atoms. Lesser charge densities provide lower bond dissociation energies (i.e. weaker bonds). Because the bond between H-S is weaker than H-O, H2S will dissociate and form ions more easily than water and is therefore a stronger acid.
3. Compare H3PO4, NaH2PO4, Na2HPO4, and Na3PO4: With respect to acidity, H3PO4 > NaH2PO4 > Na2HPO4 > Na3PO4. According to the Bronsted definition of acids and bases, acids donate protons and bases accept them. Proton donation becomes much more difficult as the resulting anions become more electronegative. H3PO4 can form H2PO4- more easily than H2PO4- can form HPO42-, etc.
4. Compare HClO and HClO4: HClO4 is a stronger acid than HClO. As the oxidation state of the central atom becomes larger, the acidity of the molecule increases. An atom becomes more electronegative as the oxidation number increases. In this case, the chlorine atom draws electrons from the oxygen atoms that surround it. This then causes the oxygen atoms to become more electronegative. The O-H bond then becomes more polar and therefore more acidic.