At 20°C, the ion-product constant of water, Kw, is 6.88 x 10-15 . What is the pH of pure water at 20°C?
A) 7.000
B) 6.501
C) 7.181
D) 7.081
E) none of these

Respuesta :

Answer:

The correct answer is D) 7.081

Explanation:

The water equilibrium is the following:

H₂O ⇄  H⁺ + OH⁻      Kw

Where Kw= [H⁺] x [OH⁻]

In pure water: [H⁺] = [OH⁻]= C

If we introduce C in the expression for Kw:

Kw= [H⁺] x [OH⁻]= C x C= C²

⇒ C= [tex]\sqrt{Kw}[/tex]= [tex]\sqrt{6.88 x 10^{-15} }[/tex]= 8.29 x 10⁻⁸

pH= -log [H⁺] = -log C = -log (8.29 x 10⁻⁸) = 7.081