See explanation below.
Work Step by Step
Polyprotic acids are acids with more than one ionizable proton. For example: phosphoric acid (H3PO4) has 3 ionizable protons, therefore it has 3 Ka values. The reason successive acid dissociation constants decrease in magnitude is because it is more difficult for an H+ ion to leave a singly charged anion (like H2PO4-) than to leave a neutral molecule (like H3PO4); it's even more difficult to the proton to be "ripped off" from a doubly charged anion (like HPO42-). All polyprotic molecules follow this pattern: Ka1>Ka2>>Ka3. The smaller the Ka value is, the easier the proton will be removed.