To calculate the hydroxide ion concentration ([OH-]) in a solution of HCl, we can use the autoionization constant of water (Kw) and the concentration of hydrogen ions ([H+]) provided.

The autoionization of water is represented by the equilibrium:

H2O ⇌ H+ + OH-

The equilibrium constant for this process is Kw, which is equal to the product of [H+] and [OH-] at 25°C.

Kw = [H+][OH-] = 1.0 × 10^-14

In HCl, the hydrogen ion concentration [H+] is equal to the concentration of HCl, as HCl completely dissociates in water to form H+ and Cl- ions. Therefore, [H+] = 2.7 × 10^-3 M.

Using the equation Kw = [H+][OH-], we can calculate [OH-]:

[OH-] = Kw / [H+]

[OH-] = (1.0 × 10^-14) / (2.7 × 10^-3)

[OH-] ≈ 3.7 × 10^-12 M

Therefore, the hydroxide ion concentration in 2.7 × 10^-3 M HCl is approximately 3.7 × 10^-12 M.