Hydriodic acid (HI), known as hydroiodic acid, hydrogen iodide, is a strong acid. It is a molecule made up of a hydrogen atom and an iodine atom. A colourless gas, hydrogen iodide HI, is dissolved in water to form hydroiodic acid, a pale yellow watery solution. Hydroiodic is a powerful reducing agent and is a highly corrosive acid.

Uses

Like hydrogen halides, hydroiodic acid adds to alkenes to give alkyl iodides. Hydroiodic acid is used to prepare iodides by reaction with metals or their oxides, hydroxides, and carbonates.

Hydriodic Acid is most commonly used as a catalyst or reducing agent in chemical reactions.

Due to its high level of acidity which allows it to kill several types of germs and viruses, hydriodic acid is often used to disinfect and sanitize medical tools and products.

In pharmaceutical applications, hydriodic acid is used as an intermediate to manufacture various medicines. 

Hydroiodic acid strong or weak

Hydroiodic acid is a strong acid with a pH of 3.01. When dissolved in water, it releases H+.  In HI iodine is less electronegative and it has large size. Therefore bonding between hydrogen and iodide is weaker. It will easily split in comparision of HF. Due to more liberation of H+ ions HI is stronger acid.