For the series of oxoacids H3PO4 , H2SO4 , and HClO4 , which acid is strongest? Why?

Answer 1

The relative strength is determined by (a) the electronegativity of the central atom and (b) by the number of oxygen atoms. Here is a good description. For this series HClO4 is the strongest followed by H2SO4 followed by H3PO4.

https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Map%3A_Inorganic_Chemistry_(Miessler_Fischer_Tarr)/6%3A_Acid-Base_and_Donor-Acceptor_Chemistry/6.3%3A_Br%C3%B8nsted-Lowry_Concept/6.3.2_Br%C3%B8nsted-Lowry_Superacids_and_the_Hammett_Acidity_Function/6.3.3_Thermodynamics_of_Solution-Phase_Br%C3%B8nsted_Acidity_and_Basicity/6.3.4_Thermodynamics_of_Gas_Phase_Br%C3%B8nsted_Acidity_and_Basicity/6.3.5._Non-nucleophilic_Br%C3%B8nsted-Lowry_Superbases/6.3.6._Rules_of_Thumb_for_thinking_about_the_relationship_between_Molecular_Structure_and_Br%C3%B8nsted_Acidity_and_Basicity*/6.3.7.__The_acid-base_behavior_of_binary_element_hydrides_is_determined_primarily_by_the_element's_electronegativity_and_secondarily_by_the_element-hydrogen_bond_strength.*/6.3.8._The_Acidity_of_an_Oxoacid_is_Determined_by_the_Electronegativity_and_Oxidation_State_of_the_Oxoacid's_Central_Atom*