Acid Strength
HI > HB > HCl >>> HF
Why? Acid strength is most largely determined by the H-X bond energy. Thus, H-I bond is weakest, therefore it is the strongest acid.
HOCl > HOBr > HOI
Why? In this case, the bond energy is the same, an O-H bond in each case. Therefore, electronegativity is the major factor. The more electronegative the halogen, the better the conjugate base is stabilized, therefore, the stronger the acid.
HBrO4 > HBrO3 > HBrO2 > HBrO
In this case, it is similar to above. The more electronegative oxygens, the more stable the conjugate base by induction, the more acidic the conjugate acid.