The strength of an acid is determined by its ability to donate protons (H+ ions). The more easily an acid can donate a proton, the stronger it is. The strength of an acid also depends on the stability of its conjugate base. The more stable the conjugate base, the stronger the acid.
In the case of HCl and HI, HCl is a stronger acid because the Cl- ion is a stronger base than the I- ion. This is because the Cl- ion is smaller than the I- ion, and therefore has a higher charge density. This means that the Cl- ion can more effectively attract protons, making HCl a stronger acid.
