If 0.30 mol of HI is dissolved in 750 mL of water, what are [H3O+], pH, and [OH-]?
HI is a strong acid and can be assumed to dissociate completely in water to give H+ and I- ions. Since the concentration of HI is 0.3 Molar, the concentration of H+ will also be 0.3 M (ignoring any concentration of H+ from the water**) pH is the negative log(base 10) of the H+ molar concentration Log 0.3 = -0.5229, so the negative log is +0.5229 so pH is 0.52 (difficult to measure more accurately). The self dissociation constant of pure water Kw at room temperature is 10 to the power -14, and this is always fixedand equals [H+] x [OH-] So if [H+] = 0.3 then [OH-] = (10 power -14)/ 0.3 = 3.3 x (10 to power -14) of which the log is -13.48 so the negative log is 13.48 so p{OH} = 13.48 {ie pKw = 14 = pH + pOH = 0.52 + 13.48} for simplicity So. [H+] = 0.33 molar; pH = 0.52; and [OH-] = 3.3 x 10(power -14) molar **The [OH-] value will also be the amount of H+ contributed by the water so you can see why we ignored it above.
