What does the acid dissociation constant (K_a) represent in a chemical solution?

How can you determine if an acid is strong or weak using its K_a value?

Calculate the pK_a for an acid with a K_a of 3.2 x 10^-7.

For a polyprotic acid, how does the pH relate to the pK_a values to determine the principal species?

In a solution of carbonic acid (H₂CO₃) with pK_a1 = 6.35 and pK_a2 = 10.33, what is the principal species at pH 8?