Question

At 25 °C, what is the hydroxide ion concentration, [OH−], in an aqueous solution with a hydrogen ion concentration of [H+] = 2.10 × 10− 3 M?

Answer 1

Solution:

Auto-ionization of water is defined as the process of formation of hydronium (H₃O⁺) and hydroxide (OH⁻) ions in aqueous solutions. 

2H₂O_(l) ⇌ H₃O⁺_(aq) + OH⁻_(aq)

The concentration of these ions can be related together through the auto-ionization equlibrium constant (K_w):

K_w = [H₃O⁺] × [OH⁻] = 10^-14

Since [H₃O⁺] is equal to [H⁺], then:

[OH⁻] = K_w / [H⁺];

[OH⁻] = (10^-14) / (2.10×10^-3) = 4.76×10^-12

[OH⁻] = 4.76×10^-12 M.

Answer: 4.76×10^-12 M is the hydroxide ion concentration, [OH⁻].