The pKa of sulfuric acid is -3. This value corresponds to a Ka of 1.0 x 10^3 as the pKa is the negative logarithm of the Ka value. Sulfuric acid is considered a strong acid because the Ka is larger than 1.
Ka and pKa provide a way to quantify the strengths of different acids. Ka is called the acid dissociation constant and is a measure of how much the acid dissociates in water. High Ka values mean that there is a high dissociation, so the acid readily donates protons. Some other common acids with high Ka values include hydrochloric acid and nitric acid, which have Ka values of 1.3 x 10^6 and 2.4 x 10, respectively.